Undertale

Everything About Fiction You Never Wanted to Know.
The friendly RPG where nobody has to die.

Undertale is a Role-Playing Game by Toby Fox, funded on Kickstarter and released on September 15th, 2015. A demo is available at http://undertale.com/, and a trailer is available here. The game is currently available to purchase on several platforms (including Steam): a port to PlayStation systems (namely PlayStation 4 and Vita), along with an official translation to Japanese, was released in August 2017; another was released for the Nintendo Switch on September 18, 2018, and a third was released for the Xbox Game Pass on March 16, 2021.

Years ago, Humans and Monsters ruled the earth together. One day, war broke out between the two races and humanity emerged victorious, sealing the monsters underground with a magic spell. In the present day, a small child playing in a cave trips and falls down an enormous hole. They wake up on a bed of flowers in a mysterious place...

Undertale draws heavily from EarthBound and Mother 3, but the core gameplay is very different from typical RPGs. Instead of selecting commands and having the computer calculate hits, each enemy attack brings up a smaller arena where the heart-shaped avatar of your soul must dodge projectiles like in Bullet Hell. In battle, the player can choose to FIGHT an enemy or SPARE them after ACTing in a way that let you do that.

The game has received critical acclaim and a dedicated fanbase has revolved around it, becoming popular among letsplayers. Currently, it is the most well-received video game made on Game Maker.

You can read more about Undertale through the Undertale Wiki, but tread cautiously, as it is chock-full of spoilers.

A spinoff game titled Deltarune was released in October 31, 2018.

Tropes used in Undertale include:

First, a very special trope:

  • Multiple Endings: There are three major routes in the game, and even mentioning their requirements is a spoiler in and of itself. This tropelist will cover only events found in the Neutral route. This page also includes examples which apply to Undertale in general, not just to any specific route. Tropes which specifically apply to the other routes can be found here:
  • Absurdly Ineffective Barricade: Papyrus's fences, which are built with posts much too far apart to stop you.
  • Acme Products: There are a lot of MTT-brand products in the underground.
  • Action Commands: When attacking, an eye-like meter will appear over the menu, and a bar will run across it. Confirm the attack when the bar is near the middle, and you do more damage. Don't press anything, and you won't attack. Most weapons change this system a bit, such as requiring mashing of the attack button to deal more damage (Tough Glove) or sending multiple bars at once (Ballet Shoes).
  • Adaptation-Induced Plothole: An extremely minor example. Since the PlayStation versions replace all dialogue referring to the F4 key, the "secret fourth frog" Brick Joke during the Playable Epilogue comes off as completely random.
  • Adorkable: Quite a few characters qualify, but this is especially the case for Papyrus and Alphys.
    • Papyrus is a skeleton with aspirations to hunt humans, but despite his choice in career, he comes across as an Ineffectual Sympathetic Villain. Even if he defeats you in battle, the worst he'll do is bring you down to 1 HP and escort you to a Cardboard Prison. Undyne even lampshades that Papyrus is too nice to be a Royal Guard.
    • Alphys is a straight-up Otaku who spends way too much time on social media and nerds out over her favorite anime. She's also a Woobie with crippling self-esteem issues.
  • Adult Fear:
    • The backstory is full of this. A long while back, the Dremurrs rescued a child that fell into the Ruins and was injured by the fall. They took care of the Fallen Child, who loved Asriel and their adoptive parents but seemed to have a nasty sense of humor about things. The Fallen convinces Asriel that to break the barrier, the Fallen would need to die, via ingesting buttercups. Asriel said it was a bad idea but reluctantly agreed when the Fallen pointed out all the monsters would be free once they got seven souls. Toriel was unable to heal their child, and the Fallen passed away that night. Asriel absorbed the Fallen's soul, ostensibly to cross the barrier and bury their body in the village but to actually gain seven souls. It didn't go well; as Asriel put it, he realized that the Fallen wanted to slaughter everyone in their village when the humans attacked Asriel as he lay the body down on the grass, mistaking him for killing his adoptive sibling. Asriel refused to fight back and crossed back over, coated in golden flower seeds, dying in his parents' garden. As the monsters on the Neutral and Pacifist Route put it, Asgore and Toriel lost two children in one night.
    • Since that night, Asgore's grief turned into anger. He mandated that any human who fell into the Underground would be killed, and their souls used to break the barrier. Toriel broke up with Asgore over this; she exiled herself to the Ruins and locked out the other monsters, to ensure that she could at least try to save anyone that ended up within the mountain. While she says she hates how cowardly he was that he didn't think to use the Fallen Child or another human's soul to cross the barrier and take human souls, Toriel is not a killer. She tells you that she saw five children leave the Ruins, and none of them survived. (She's right; you can find their items strewn across the four areas). If you manage to spare her and leave, Toriel realizes that you are the seventh soul but in good conscience, she can't let you kill Asgore or Asgore to kill you. The True Pacifist ending has her stop the fight before it begins; while she's mad at Asgore for killing her adoptive children, she asserts that no one has to die, and they can live happily.
  • All Lowercase Letters: Frequent in characters, and used as a shorthand for depression, judging that the characters more prone to it are Napstablook and the self-deprecating Alphys. Sans seems to be the exception, as he is part of the all lowercase speech group, but his Meta Guy qualifications make credible that he is so lazy he just doesn't bother capitalizing things. Except that he is revealed to be as depressed and self-loathing as the others.
  • Animalistic Abomination: Endogeny, one of the Amalgamates, takes on the vague shape of a large canine: it has two pointed ears atop its "head", a single vaguely mouth-like orifice where a face would be that produces "Happiness Froth" when it's excited, and a wide body with six digitigrade legs that form the silhouettes of five smaller canines, which display satisfied faces once it's content. For all its weirdness, though, it's very much like a normal dog, and treating it like one by playing in a specific way with it is the key to Sparing it.
  • Animal Motifs: While Toby Fox never physically appears in the game, his avatar does. A Pomeranian known as the Annoying Dog, he primarily bothers Papyrus and notably absorbs the Legendary Artifact. He can also be found as an Easter Egg after the games credits as, sleeping in a room representing his office next to his computer.
  • Apocalyptic Log: Two of them, both in the same hidden location. The True Laboratory has the written logs from Alphys and the audio tapes from before the deaths of Asgore's children.
  • Arc Words:
  • Arson, Murder, and Jaywalking: At one point in Snowdin, Papyrus sets up a gauntlet of traps including such dangerous weapons as a cannon, a flamethrower, a spiked ball-and-chain, a spear and... the Annoying Dog dangling from a rope.
  • Ascended Fridge Horror: Toriel at the beginning prepares to destroy the entrance to Snowdin because every child that has fallen into the Ruins goes out, dies, and gets their SOUL harvested by Asgore. Before that, however, they spent enough time with her to outgrow their shoes, win her love, and evolve her into the My Beloved Smother that you encounter. She says that she saw every child that fell into the Ruins die eventually.
  • Bad Guy Bar: In Snowdin Town's local restaurant Grillby's, you can find all 5 of the dog mini-bosses you encountered along the way (assuming they're alive, of course). Subverted in that they don't treat you like an enemy anymore and talk to you normally.
  • Be Careful What You Wish For: Burgerpants says this word for word, after admitting that he initially came to Hotland because working with Mettaton was his greatest dream. Now that he does work for Mettaton, he realizes he's a Bad Boss that somehow coasts entirely on Popularity Power despite the MTT Resort being, in Burgerpants's words, "a labyrinth of bad choices."
  • Behind the Black:
    • The "conveniently-shaped lamp" the player hides behind in the first scene with Papyrus only covers the player from the camera's point of view; given that Papyrus was standing to the side, he should have easily been able to spot where the player was standing.
    • There's a crystal formation in waterfall that the protagonist finds particularly beautiful, but it's on the southern wall. The only hint of this formation existing is calling Undyne and Papyrus in the area.
    • This is a core feature in the perspective puzzle in the Ruins, in which coloured switches are hidden from the player by pillars, but would be perfectly visible to the character.
  • Better Than a Bare Bulb: If there is a JRPG or general video game cliché present in the game, it will be commented on. Either it'll be for a quick gag, or it'll become a major Deconstruction that the game ends up revolving around.
  • Betty, Veronica and Archie Switcheroo: Parodied: when you go on the date with Alphys during the route to the Golden Ending. You by default are the Betty while Undyne is the Veronica and Alphys is the Archie. Thing is you can be a rude pacifist, or have committed a No Mercy run beforehand. She tries her best to make it work, but Alphys admits that she has feelings for Undyne, the one who actually wrote the letter that you delivered. You can then choose to roleplay as either Undyne or Alphys, making the real Alphys blush just in time for Undyne to walk in, giving Alphys the courage to confess her feelings.
  • Big Bad: Asgore Dreemurr, king of the Underground's monsters. His goal is to gather seven human SOULs and shatter the barrier keeping the monsters locked in Mt. Ebott so they can finally be freed from their imprisonment and exact their vengeance on humanity. As it turns out, he's more of a Tragic Villain, and only wants to end the monsters' oppression, but feels that a peaceful resolution is simply unattainable by this point. In fact, it seems that Flowey is a better candidate for the title.
  • Big Bad Ensemble: Mettaton and Asgore. Asgore is king of the monsters and is the overarching threat of the game, while Mettaton is simply a superstar robot who, however, is responsible for many of the bosses you fought over the game and is revealed to have independent goals from Asgore, appears more than Asgore does and has more presence. It's only after Mettaton's defeat that you fight Asgore finally. Flowey the Flower is also with them and is the final big bad of the game.
  • Bitch in Sheep's Clothing:
    • Flowey the Flower seems like an innocent, adorable little fellow, but right after meeting them, they try to trick the player into getting seriously hurt. After their fake tutorial, they then attempt to murder you while breaking into a fit of maniacal laughter.
    • While not a malevolent example, Doctor Alphys fits the bill. Sure, there were no intentions of ever putting you through real danger, but she still sent a killer robot after you and reactivated a bunch of traps and puzzles that you would have to pass through so she could swoop in and play hero in order to boost her non-existent self-esteem.
  • The Blind Leading the Blind: Undyne admits, after giving a violent cooking lesson that leaves her kitchen a mess and her house in flames, that she now understands why Papyrus, her original student, sucks at cooking.
  • Bloodless Carnage: Monsters turn to dust when killed, so no blood is spilled. Subverted (or Averted, depending on player interpretation) once in the Genocide run, as mentioned above. There is some evidence to suggest that monsters shed blood when injured prior to turning to dust, however, as also noted above, but since the only example we see is from a Boss Monster (a very powerful kind of monster with unique biology even by monster standards), nothing can be said for sure.
  • Boke and Tsukkomi Routine: Papyrus and Sans, respectively.
  • Boss Banter: Many bosses are fond of talking to you during their battle (as are regular enemies, for that matter), but the final bosses of each of the routes fit best. The Neutral one gloats about killing you and your friends, while the other bosses sound more like they're ranting on their personal soapboxes than trying to kill you.
  • Buffy Speak: Sans calls Papyrus's fence in Snowdin Forest a "gate thingy".
  • Bullet Hell:
    • Enemies attack with bullets of varying shapes and patterns. Battles with multiple enemies can become hectic.
    • Beware that some of the attacks will come from out of the square your heart is in.
  • Burger Fool: MTT-Brand Burger Emporium, even down to the mandatory slogans. Management is incompetent in several respects and outright sadistic in others, alternatingly micromanaging and operating entirely on whims. The leitmotif is the same pitched-down version of "Shop" you'd hear in other stores during a Genocide run, no matter which end you go for.
  • Cardboard Prison: Papyrus's attempt to use his and Sans's shed as an impromptu prison for the player is quite unimpressive to say the least. His only measure for hindering escape attempts is placing a fence across the room, which has such large gaps between the bars that it can be circumvented by walking out between them, and the door to the shed turns out to be locked from the inside.
  • Collectible Cloney Babies: Alphys collects merchandise from the Kissy Kissy Mew anime franchise. In the Switch version, you can even get into an optional boss battle with one at the Dog Shrine.
  • Color-Coded for Your Convenience:
    • Some bosses can change the color of your SOUL, which changes the way its movement works. Red is the default and can move freely. Blue subjects your soul to gravity, dragging it to the ground and forcing you to jump to move vertically. Green prevents you from moving, but gives you a shield that you can point in different directions to block incoming attacks. Purple forces you to hop between horizontal lines for vertical movement, but still allows you to move back and forth along those lines. Yellow allows your soul to shoot projectiles. All of these changes disable the FLEE option.
    • Enemy projectiles have colors as well. White projectiles are normal and do damage when touched. Light blue attacks won't hurt you as long as you aren't moving, while orange ones will only hurt you if you're staying still. Green attacks will heal you when you touch them and/or must be touched to spare an enemy. Grey attacks (only so far used by ghosts/objects possessed by ghosts) do nothing at all. They're used to relay messages in a non-harmful manner. Red attacks, similar to grey attacks, are often used as a warning in order to allow you to know when an attack is coming.
  • Context Sensitive Button: The ACT option in battles. When it is selected for an enemy, a set of options unique for them pops up, and you can choose any one of them. The effects they have include doing nothing, changing your stats or the enemy's stats, affecting their next attack, allowing them to be spared or making them leave the battle, and any number of miscellaneous effects depending on the enemy.
  • Continuing Is Painful: Averted for the most part, except during Papyrus' fight. Unlike most battles, where you die and are sent back to the last place you saved, it's completely impossible to die during Papyrus' fight as he captures you, and the battle ends once you hit 1 HP. While this might seem like a good thing, this means that after every fight the healing items you used in the previous fight are still gone, and in subsequent tries, you'll have to waste money to buy more healing items.
  • Convection, Schmonvection: Zigzagged. While Undyne's armor heats up significantly while crossing a bridge over lava, to the point where she passes out, you are perfectly fine in that same area and suffer no heat related issues, even when you make it hotter to appease a monster. Then again, Undyne is a fish monster wearing heavy steel armor. Your character is wearing a simple striped shirt. Also, the Royal Guardsmen apparently have "cooling dirt" on their armor, which you need to polish away in order to get one of them to succumb to the heat and take his armor off as well. Since Undyne is normally stationed in Waterfall, her armor would have no need for such enhancement.
  • Cute Monster Girl: This trope is more incidental than anything; the majority of monsters, male or female, are cute to some degree.
  • Deadly Euphemism: Throughout your journey, you'll hear about monsters who have "fallen down". It's revealed through Alphys' entries in the True Lab, and in Snowdin library, that "fallen down" means "comatose and near death".
  • Dem Bones: Sans and Papyrus are an interesting case: they look like human skeletons, yet they are actually a species of monster. There is no mention of them dying in the past, implying that they have always been skeletons. Supported by Papyrus thinking that humans "descended" from skeletons, showing that their skeletons are different.
  • The Dev Team Thinks of Everything: Undertale is a very clever game, where a lot of dialogue changes depending on player actions, down to some incredibly minor ones.
    • One example: there are over 70 variants of the Neutral ending, wich depends on how many monsters were killed, which and how many bosses were spared, and whether the player keep their original equipment or not.
  • Diagonal Speed Boost: The game doesn't reduce your horizontal or vertical velocity if you move diagonally. While this can be beneficial on the map, it can feel awkward in battle, particularly for players of Shoot'Em Ups that are used to this trope being averted. Fortunately, it can be disabled in the options menu.
  • Difficulty Spike: The Pacifist and Genocide routes are markedly harder than playing normally in the Neutral route. In the Pacifist route, you must avoid killing absolutely everything, which means you will not gain any attack power or HP and will have to get really good at dodging. In the Genocide route, you have to kill absolutely everything to get super strong, which means lots of grinding to purge all random encounters. The few bosses that can pose a challenge are absolutely brutal; the game will get really easy as you gain EXP and get stronger, till you can one shot most enemies, including bosses, but two bosses are still capable of giving you a good fight, as they're the toughest in the game.
  • Disc One Nuke: You can get the Temmie Armor before you are even halfway through the game, though this requires an enormous amount of money. Not only does it have the highest defense value of any armor in the game, it also increases your attack, the invincibility frames after getting hit, and restores one point of health every turn in combat.
  • Double Unlock: The Temmie Armor requires you to pay one thousand gold to get the option to buy it, then a very high amount of gold to actually obtain it.
  • Do Well, But Not Perfect: In Thundersnail, if you win, you earn 9 G, which is less than the entry fee of 10 G! Napstablook explains that they have to make a profit somehow. If you lose by a very narrow margin, Napstablook will instead give you 30 G to avoid disappointing the snail you were cheering on.
  • Down in the Dumps: Partway through Waterfall, you fall into the Trash Zone, which, fittingly, is filled with garbage. And a mini-boss. You return there for Alphys' date.
  • Dreamworks Face: One of the sprites in the game files shows Toriel making this face and is named spr_face_torieldreamworks_0. Papyrus also has a confident expression during the battle with him.
  • Early Installment Weirdness: The Kickstarter videos have some different characterizations from the actual game. Sans is seen closing his mouth to drink lemonade, while he never closes his mouth in the game, sporting a goofy grin at all times. Toriel is also shown manifesting lemonade in thin air, though the only magic she's shown to have in the game is fire magic.
  • Earn Your Happy Ending: The way to get the True Pacifist Endiong. If you spared every enemy and got to the final room with no kills, not only did you not increase your LV (meaning you beat the game on the starting 20 HP), you still don't get the good ending unless you've befriended everybody. Then after that, you go through the True Lab, and found out the secrets of the hidden lab, as well as the truth of Flowey's existence, followed by another boss battle.
  • Easter Egg: There are plenty! But they're spoilerly enough to have their own page.
  • Easy Levels, Hard Bosses: To a heavy degree. Standard enemies, while not effortless, can usually be spared either instantly or after a single ACT, with only a few exceptions, and attacking them will bring them down pretty quickly, especially if you've gotten good with weapon timing and have higher LV. Bosses (at least the ones from Papyrus and beyond) are another story: they have high health, they employ Bullet Hell with their attacks, they mix up the dodging system, and sparing them requires either a thoughtful series of actions or dragging the fight out for quite a while (over 20 or more turns for the major bosses; most enemy battles will take 2-4 turns at the most). This is especially true in the Genocide path: you can plow through enemies with ease, but anything that can take more than a hit from you is far harder to beat than anything on other routes.
  • Egopolis: In the Neutral ending where Mettaton becomes the new ruler of the Underground, he turns it into this.
  • Either/Or Prophecy: The Delta Rune in Waterfall tells the prophecy that an Angel will come down from the mountain and "the Underground will go empty." Either the Angel will destroy the barrier and free monsterkind from imprisonment, or the Angel of Death will slaughter everyone.
  • Eldritch Abomination: Omega Flowey and the Amalgamates.
  • Empathic Environment: The save point immediately before the battle with Undyne reads "The wind is howling. You're filled with determination...". If you kill her, the wind stops, and its message changes accordingly.
  • Eternal Engine: The CORE, a high-tech center that serves as the power source of the Underground. When the player arrives, it’s been taken over by Mettaton, who leaves traps and mercenaries everywhere.
  • Everyone Can See It: During Mettaton's quiz, he asks who Alphys' crush is. If the player responds "Undyne", she blushes, and Mettaton says "I told you it was obvious. Even the human figured it out." Even more so in her date sequence where she attempts to offer you metal polish, scale cream and a spear repair kit. One guess for who those are meant for.
  • Everything Fades: Monsters turn to dust when killed, explaining why they vanish after being fought. This is notably incorporated into monster funerals and comes up a few times in the plot of the game.
  • Evil Is Easy:
    • EXP can only be gained by fighting enemies; sparing them only gives you gold. Of course, this means that if you don't fight anyone, your life bar will never get bigger...
    • Taking the full Genocide route eventually subverts this. The amount of encounters in this route that don't go down in one or two turns can be counted on one hand. But those encounters are the most challenging in any route of the game.
  • Fake Trap: The corridor of spikes can't harm you because the spikes that aren't part of the correct path only act as barriers. You wouldn't realize this at first because Toriel leads you through.
  • Fantasy Kitchen Sink: The setting of Undertale is a place where you can meet a pair of comical skeleton brothers, a genocidal monster flower, a middle-aged pie-baking goat lady, an anime geek dinosaur scientist and a Wrong Genre Savvy Hot-Blooded fish knight.
  • The Ferry Man: The River Person, who, after you reach Waterfall, will allow you to quickly travel between Snowdin, Waterfall and Hotland, the game's three main areas.
  • Flawless Victory: If you dodge all of the names in the special backer credits, you'll gain access to the Developer's Room.
  • Flower Motifs: Golden Flowers, or Buttercups.
  • Foolish Sibling, Responsible Sibling: The trope is played with Sans and Papyrus. At first look, you may pin Sans as the Foolish (what with being completely committed to slacker lifestyle) and Papyrus as the Responsible (being the housekeeper, constantly reminding Sans of his responsibilities), but there are details that muddies the issues, like Papyrus' actual personality being actually quite naive and childish, while Sans seems to be the actual breadwinner of the household and is more responsible that he initially seems.
  • Four-Temperament Ensemble: The four monsters you befriend in the Pacifist route. Papyrus (sanguine), Undyne (choleric), Alphys (melancholic) and Sans (phlegmatic).
  • Fragile Speedster: If you keep the Bandage on, you'll have no additional defense whatsoever, but you'll be guaranteed to be able to run from any encounter that gives you the option.
  • The Friend Nobody Likes: Jerry, a deformed, butt-ugly monster who has a random chance of appearing in one of the enemy mobs in Snowdin's outskirts. All he does is act boorishly and annoyingly towards the other monsters, who will celebrate with the player should you choose to ditch him.
  • Frying Pan of Doom: The Burnt Pan weapon, which is even more useful on a Pacifist run since wielding it causes consumable healing items to restore 4 more HP than normal.
  • Furry Confusion:
    • A rabbit lady in Snowdin has a small, non-anthropomorphic rabbit on a leash as her pet. A rabbit guy nearby finds this a little disturbing. The Playable Epilogue reveals that this leash-rabbit is actually her brother.
    • Catty also talks about how she wants to buy a cat at some point despite being a big humanoid cat herself.
  • Furry Female Mane: Gender-inverted with Asgore and Toriel. While Toriel doesn't have feminine hair, and her only Tertiary Sexual Characteristics are eyelashes, Asgore spots a long viking-like blonde hair and a beard to highlight his manliness.
  • G-Rated Drug: Doggo likes to smoke dog treats.
  • Game Over Man: Under certain circumstances, the Game Over Man is different from the usual:
    • Being defeated by the Neutral Final Boss causes him to taunt you during the Game Over screen after Asgore says his usual line.
    • During the Genocide Final Boss, trying to spare the boss at a point will cause him to kill you. The resulting game over screen will have "Dogsong" play instead of the regular Game Over tune while the boss will assert that, after all of your genocidal actions, he refuses to allow the battle to resolve peacefully.
  • Gameplay Roulette: While the game is mostly presented as a JRPG, each enemy you battle in Undertale has at least one unique bullet hell-style attack that you must defend against, and each one has a unique text-based "puzzle" you must solve in order to spare them. In addition to this, the Underground itself is absolutely covered in puzzles that must be solved in order to get from place to place, and the game tends to take on horror-like undertones at the end of each route. There are also short mock-dating sim segments that the player can do with a few of the characters.
  • Genre Roulette: Most routes of the game are primarily an adventure/comedy, but with more than their fair share of tragedy and horror thrown in, often with very little warning. Take the Genocide route, however, and the game becomes so relentless and unapologetic in these latter elements that it's barely recognizable as the same game anymore.
  • Go Look At the Distraction: During a Neutral/Pacifist run battle with Mettaton, Alphys instructs you to turn Mettaton around so you can flip the switch on his back, and you do so by telling him there's a mirror behind him. If you lose to Mettaton and return, you can opt to skip his speech and turn him around straight away by telling him there's "something cool" behind him.
  • Good Feels Good: Whenever the game isn't judging and/or guilt tripping you for being a jerk, it's instead encouraging, supporting and appreciating you for being good, or even just not being bad, and it's very good at making you care about your choices.
  • Half the Man He Used To Be:
    • If you kill Toriel, she will have visible slices on her body and in her face.
    • In the Genocide route, Undyne takes a blow for the Monster Kid that visibly cuts her battle sprite in two, from shoulder to hip. Her upper half begins to slowly fall off... until she reforms herself for the real boss battle.
    • The final boss of the Genocide route has his torso diagonally slit in half all the way from his shoulder to his hip.
  • Hand Wave: Whenever the player is slightly out-of-bounds (when it should be impossible), "magic glass" appears underneath them so they won't be walking on air.
  • Harsh Life Revelation Aesop:
    • Asgore's story shows how your anger can destroy everything. When the humans killed Asriel, Asgore and Toriel's son, shortly after they lost their adopted human child, Asgore in a fit of grief declared that all humans who come to the Underground will be killed and their souls forfeit to open the barrier. This ends up driving Toriel away to the Ruins, and soon a few bodies pile up, much to Asgore's horror. He can't rescind the policy, however, because the Underground needs the hope that they will be free.
    • As the Golden Ending shows, some mistakes are plain unforgivable, something that Toriel states. She's still mad at Asgore for declaring the "Kill All Humans" policy but not having the courage to go to the surface and kill six more humans, which would have saved decades of imprisonment for everyone. Toriel would have preferred to stay Underground and find another way. Though Toriel doesn't believe her ex deserves to die, she turns down his efforts to reconcile with her and spends most of her game time glaring at him.
  • Have We Met?: When you restart the game, any of the main characters that you had befriended in a previous playthrough will vaguely remember you. For example: when Toriel asks what your favorite is between cinnamon and butterscotch, she will instead take a guess, which will be whatever you picked in the last playthrough.
  • Healing Checkpoint: They also serve as save points.
  • Heartbeat Soundtrack:
    • A heartbeat plays in background of the final scene of any Genocide playthrough, where the player has killed every single NPC the game allows.
    • In "But the Earth Refused to Die": if you listen closely, you can hear the hearts of everyone in the world beating as one.
  • Hello, Insert Name Here: As is typical for any game in the JRPG genre, you start a brand new game by naming the character. It then proceeds to subvert the trope, by making every monster in the underground refer to you as "Human" or "Kid" or other various generic terms of endearment (your chosen name still appears in your status and in the battle screen).
    • There is a twist, however: nobody calls you by that name because the name you inserted is not necessarily your character, but it could be a certain different human who fell into the underground. This is revealed near the end. You might have noticed by then that the name prompt when you started the game wasn't lying to you because it said "name the fallen human", NOT "name your character".
  • He Who Fights Monsters: A major theme of the game.
  • Hidden Depths: Every major character has more to them than meets the eye. However, due to the game's extreme level of sensitivity to player choice, it's literally impossible to explore them all in a single run. In particular, going full genocide results in a very strong example of The Apocalypse Brings Out the Best In People, exposing depths that would only be hinted at otherwise. On the flipside, just about everything about Flowey is kept hidden from the player until the end of a True Pacifist run.
  • Hidden Track: Only the first 77 tracks out of 101 of the soundtrack are available for preview on Bandcamp. The rest are hidden due to their spoiler-laden nature, and can only be legally listened to by purchasing the soundtrack.
  • Hold the Line: Some bosses and encounters take longer to finish if you're trying to spare them rather than fight. The player needs to keep hitting SPARE (and/or use ACT) and withstand waves of attacks until the boss is ready to show mercy (or accept it).
  • Holiday Mode: A minor example: if you've gotten the True Pacifist ending, the Joystick Config menu will be different depending on whether it's spring, summer, autumn or winter.
  • Hopeless Boss Fight: There are several in the game (though most are technically minibosses), and every foe that uses this trope also plays with it differently.
    • The first Hopeless Miniboss Fight is Napstablook the depressed ghost. You can attack them if you like, but once their HP falls below 50%, they reveal that attacks don't actually hurt them because ghosts are incorporeal; they were lowering their HP bar just to be polite. Then they leave.
    • The next Hopeless Miniboss Fight is the Mad Dummy, and is probably the foe that zig-zags the trope the most.
      • They also can't be hurt because they are a ghost, though unlike the foe above, this one doesn't even pretend to be fazed by your attacks. You can, however, progress the fight by luring their minions' own bullets toward their sprite, causing them to become more and more agitated until they fire their minions out of frustration.
      • After that, they then pull out their ace-in-the-hole, a machete! ...which ends up being a slow and easy-to-dodge attack. Once they realize they're out of attacks, they stubbornly refuse to end the fight, content to just keep you trapped in the battle screen forever. Thankfully, Napstablook swoops down in the nick of time with their crying attack. Mad Dummy decides that it's just not worth the trouble, and floats off.
    • Each encounter against Mettaton the entertainer robot ends up being a single-subversion of the Hopeless Boss Fight. Because he is made of metal, his body is immune to attacks (and if you Check this foe each time, the game snarkily points out that this fact has not changed between encounters). However, each encounter ends with Alphys turning the tables on him through different means than simply killing him. Though once he reveals his EX/NEO form, you can finally kill him for good sould you choose.
    • Finally, there's the Neutral route's final boss Photoshop Flowey. Unlike the previous examples, he actually does take scratch damage to start with. And when you call for help from the six human souls, the boss' defense drops to 0, and you get regular healing, allowing you to wail on him without mercy. After you deplete his HP fully though, he just reloads his save from the beginning of the fight, and then just keeps reloading as he one-shot kills you again and again, making it thoroughly clear that Flowey has just been toying with you the whole time.
  • Horrible Judge of Character: Several monsters who assumed you were evil and attempted to kill you in a Neutral or Pacifist run will, in a Genocide run, when the character is genuinely evil, instead be incredibly trusting of them and simply offer to let them go without a fight, only to be murdered in cold blood. Justified in that if you have been sparing everyone, you will continue to look like a human, but if you have been killing everyone, you slowly start to look monstrous. Humans, as per the narrative, are the enemies of the monsters.
  • Humans Are Special: Humans, unlike monsters, are mostly physical matter, making a human much stronger than an equivalent monster. Additionally, human SOULs linger after death; this allows monsters to steal their SOULs and use them as a power source, making the monster incredibly powerful. Most importantly of all, human SOULs possess a natural abundance of Determination, which is absolutely essential for your quest to empty out the underground, however you choose to do so.
  • Humans Are Warriors: If you call her outside Gerson's shop, Undyne hints that the war between humans and monsters was a Curb Stomp Battle thanks to this. It's easy to believe when you remember a single human child is capable of taking down the monster world's greatest warriors by themselves.
  • Hyperactive Sprite: In battles, most enemies have a simple animation they repeat on loop, with only a few (most noticeably Toriel and Papyrus) that don't animate. It's lampshaded in a Pacifist victory against the Royal Guards, where 01 tells 02 how much he loves "bouncing and waving [their] weapons in sync", which, indeed, is what they do during the battle. It becomes downright surreal with Temmie, whose sprite constantly vibrates with excitement and whose face will eventually vibrate free off her body, and the closer Mettaton gets to attacking with his core for the second time in the main phase of his last boss fight, the faster he starts dancing, to the point of changing poses on every half beat of his theme.
  • Hypocritical Humor: If Mettaton catches you swearing in your essay answer, he delivers this line: "Oh my! This is a family friendly TV show." [ratings drop 150 points] "Now stand still while I murder you."
  • I Ate What?: Plenty of the food items in the game are made of questionable ingredients. Hot dogs made of water sausages, burgers made of sequins and glue, steak made of anything but meat, junk food made of actual garbage...
  • I Love the Dead: Played for Laughs during Mettaton's Quiz. One of his questions is "Would you smooch a ghost?", and ALL the answers are "Heck Yeah!". This becomes more significant after you learn that Mettaton is probably Napstablook's cousin (thus, a ghost) in a robot body.
  • I'm a Humanitarian:
    • The Vegetoid enemy is a sentient vegetable. After weakening them with attacks, you can take a bite out of them to recover HP. This was played even straighter in the demo, where you ate the whole enemy... that counted as murder, but the final version doesn't, due to some changes in design.
    • Spider cookies and spider cider, made with real spiders, by spiders, for spiders! Muffet herself pours you a cup full of spiders during the very boss fight with her. Of course, real-life spiders are often cannibalistic, so this is totally normal for them.
  • I Need to Go Iron My Dog: "I need to go to the bathroom" seems to be a catch-all for monsters excusing themselves but not having anything better to say, since monster food doesn't go farther than the stomach. Papyrus does it to excuse himself, then proceeds to jump out a window and run off, and Alphys does it and runs into another room to avoid having a panic attack (and it also disguises the fact that it's an elevator to the True Labs).
  • Inventory Management Puzzle: To a degree. The child can only carry eight items at a time, not counting their current armor and weapon. This will fill up fast because of stockpiling healing items. To help out, the game provides a "Dimensional Box" in numerous locations that allows you to swap out items whenever you want, which is useful for storing specific foods for later events or for old weapons until they can be sold. The limitation of the box (it's only in a few select locations) becomes moot with Alphys' cellphone upgrade, which gives you permanent access to the box and a second one via the phone (though not during battles); you can't get this in a Genocide run, but several more Dimensional Boxes appear near the end of the game as a compensation.
  • Irony: A game that is heavily critical of replaying it makes it, by far, one of the most replayable games ever. One might ask, "What if I spare this boss?" "What if I spare that boss?" "What if I do a Genocide run after a Pacifist run?" "What if I do two Genocide runs?" "What if I do two pacifist runs?" etc, etc. Though given the game's overall message, this could be completely intentional by making you choose: honor Flowey's last request to let the happy ending be, or reset everything to scratch the itch?
  • Kent Brockman News: Mettaton runs a sketch like this, featuring you.
  • Klingons Love Shakespeare: Because of all the stuff that flows down from the surface world, monster culture has absorbed a lot from human culture, to the point where it's pretty much impossible to say where one stops and the other begins. Most notably, Toriel has a brand-name chocolate bar in her fridge, and both Alphys and Undyne are very dedicated anime fans.
  • Leave Your Quest Test: Toriel wants you to stay with her at home. She genuinely means well by it though.
  • Leitmotif: Nearly every song in the game draws from a small pull of recurring melodies.
  • Lethal Lava Land: Hotland, the last major region of the Underground, is dry, volcanic and has many fire-based enemies. It also has some Eternal Engine elements, given that Mettaton has reign over it.
  • Literal-Minded: A fishlike NPC in Grillby's tells you he "put out a line" for some girls and is taking the "plenty of fish in the sea" phrase literally. The fishing pole can be found north of the ice room with the save point, and there's a note with a phone number at the end of the line as bait.
  • Loose Canon: Which of the three endings is the canon ending? Simple, according to Word of God, whichever one the player interprets it to be.
  • Made of Magic: Monsters are described as having bodies made mostly of magic, as opposed to humans' bodies being mostly water.
  • Manly Gay: The duo of Royal Guards stationed at Hotland are this to a T, especially the guard who speaks like a stereotypical "bro". Defeating them peacefully has you get them to confess their feelings to one another.
  • Mercy Invincibility:
    • After you get hit by an enemy's projectile, you'll have a few frames of invincibility. This duration can be increased by specific items.
    • However, the final boss in the Genocide route does not feature any mercy invincibility, as you obviously deserve none by that point.
  • Mood Whiplash: Undertale undergoes this near the end, several times. It ends up being not as lighthearted as before, to say the least. It becomes tragic when you make your way through New Home. Then after you defeat Asgore, Flowey causes the mood to once again shift quite dramatically.
    • A minor one occurs in the MTT resort, during the Dinner with Sans event. The scene is more melancholic than most of the interactions with the character to that point, and finish with a brief vision of an eyeless, extremely serious Sans before coming back to his jokester personality.
  • Murder Is the Best Solution: Flowey seems to think so, and advocates it. In gameplay, however, unless you're majorly overlevelled for the area you're in, most fights take just as long to kill someone than they do to spare them (about three to five turns, depending on the enemy setup), making killing, even in self defense, unjustified.
  • My Beloved Smother: When you first meet Toriel, it seems that her character is being built up as this... and it is, in the most benevolent sense. She's just a sweet and overly-protective old monster lady with no ill will at all.
  • My God, What Have I Done?:
    • If Toriel manages to kill you, the screen will show her making this face before the Game Over screen hits. Mind this is hard to achieve, that you would have to be running directly into the bullets.
    • This is how the player is expected to feel when a certain skeleton stands before you in the Final Corridor, and proceeds to lay down the truth for you. That EXP you've been collecting to raise your LV? Those were Execution Points, and you were increasing your Level of Violence through acts of murder.
    • This revelation could occur much earlier for the player; if you decide to grind in an area an end up killing a lot of monsters, your next encounter won't have any monsters, only a message indicating "there's no one left". The area music then turns into a creepy low-pitched rumble. Since it's an RPG, initially, you might think that killing monsters during random encounters, and doing it a lot in order to LV up, is something that's expected of you to do. But if you take it too far, you'll realize your mistake... The game itself will make sure of it.
    • This also turns out to be how Asgore feels about declaring war on humanity as revenge for his children's deaths. Normally a peace-loving, friendly fellow, he made a horrible decision due to a brief moment of grief and rage, and he has no way to take it back now that his subjects expect him to aquire the human souls needed to break down the barrier and slaughter the humans on the surface. It's shown when you meet him that he's been eaten with regret for it ever since.
  • Non-Action Guy: Most enemies in the early stages aren't aggressive, and some of them are very easy to spare. Given Flowey's claims of "kill or be killed", this would appear to be very atypical of the underground. Or so one might think from the demo alone. Monsters on the outside aren't much less docile than the ones inside. Flowey may have simply been trying to make you think otherwise.
  • Noodle Incident: If you call Papyrus and Undyne in the Apron room in Hotland, they'll reminisce about a time they "cooked" popsicles. All they reveal to you is that whatever they did created such a mess that it "took forever to hose everything off."
  • No Smoking: Typically averted, most prominently with Burgerpants, except in one instance for comic effect: Doggo smokes dog treats in the same way you would smoke a cigarette. You can even find some dog treat "butts" on the ground after you fight him.
  • "Not Making This Up" Disclaimer: A sign before a miniboss fight in the second area reads "Warning: Dog Marriage". If you check it again to make sure if you read it correctly, the text confirms that "yes, you read that correctly."
  • Obfuscating Stupidity: Sans the Skeleton fits this to a T. Like everyone else you meet, he comes off as incredibly goofy and impossible to take seriously, which is why it's all the more startling when he takes off the metaphorical mask and speaks frankly with you.
  • Occam's Razor: The factor resulting in the acceptance of certain theories such as Gaster being Sans and Papyrus's father or the Narrator Fallen Child.
  • Official Couple: Undyne and Alphys in the true ending.
  • Old Save Bonus: In order to even start the Pacifist or Genocide routes, you have to complete the game on this route first.
  • Ominous Fog: There's a heavy fog east of Snowdin that completely blocks visibility. Papyrus fights you the first time you walk through.
  • Our Ghosts Are Different: Ghosts in Undertale are just another type of monster, although they have all of the typical abilities of ghosts in fiction. SOULs are something else entirely (see below).
  • Our Monsters Are Different: "Monsters" are beings made of magic. They start at Funny Animals and end somewhere short of Lovecraft Lite abominations, with the occasional thing you can't really classify (like a tsundere airplane) in between. However, they're all more or less friendly if you get to know them.
  • Outside the Box Tactic: It's possible to completely avoid having to fight Muffet. The main way would be to buy an item from her Spider Bake Sale when you encounter it in Hotland, except the treats cost literally as much money as you can possibly carry. However, the Ruins has a branch of the sale that sells treats much cheaper. If you buy a spider treat, then eat it during the fight with her, she'll let you go right away.
  • Overly Long Title: Some of the soundtrack tracks, including "Bird That Carries You Over A Disproportionately Small Gap", "Can You Really Call This A Hotel, I Didn't Receive A Mint On My Pillow Or Anything" and "Song That Might Play When You Fight Sans".
  • Overly Narrow Superlative: During a phone call with Papyrus while in Hotland, he states that Mettaton is his "favorite sexy rectangle".
  • Pachelbel's Canon Progression: Appears partway into "Fallen Down (Reprise)", specifically at the part that quotes the game's main theme.
  • Pacifist Run: Not only possible, but the only way to get the Golden Ending is to not kill anyone during the game.
  • Peer Pressure Makes You Evil: The Migosp attacks only while it has allies present, but acts like itself (a carefree dancer) once they're gone.
  • Petting Zoo People: A large chunk of the monsters are basically anthropomorphic animals (goats, cats, bunnies, fish, dogs, etc) with human personalities and clothing.
  • Pistol-Whipping: Since the Empty Gun has no bullets, it can be inferred that this is how the weapon is wielded when you use it.
  • Planet of Steves: Temmie Village. Apparently, all Temmie have "Temmie" as a name. Except for Bob, that is.
  • Platform Hell: Most encounters tend to play as Bullet Hell, but some require extensive platforming, especially the final boss of the Genocide run. You're gonna have a bad time indeed.
  • Playable Epilogue: In the Golden Ending, you can go back and talk to the NPCs around the world. Like in EarthBound, they will have unique dialogue to share; most of it isn't too important, but back-tracking to the very first room in the game gives you a bit of a surprise.
  • Player Tic: People doing Let's Play of this game tend to read everyone's speech bubbles, with or without assigning different voices/inflections to everyone.
  • Playing a Tree: During the ending credits, Burgerpants finally gets his chance to be on stage with Mettaton. Unfortunately, it's as a bush.
  • Protagonist-Centered Morality:
    • Subverted big time. Spare the major encounters but kill common monsters on the way? You'll get called out on it.
    • Lampshaded by Alphys: "Watching someone on a screen really makes you root for them."
  • Puzzle Boss: On a Pacifist/Neutral run, all encounters essentially become this, with the 'puzzle' being working out how to end the encounter non-lethally.
  • Rainbow Speak: As standard for RPG, but Undertale has some interesting uses.
  • Random Encounters: The protagonist shows a "!" speech bubble just before an encounter starts. Initiate the Genocide run, and nobody shows up in the encounter. The speech bubble also changes to a smiley face similar to Flowey's.
  • Random Event: The "fun" value in your save file is set to a number between 0 and 100 when you start a new game and depending on what it is, there's a usually a 50% chance that you'll see an event corresponding to that number in the location where it can occur.
  • Rated M For Manly: Should you wish, you can flex at certain monsters, one of which (the merhorse Aaron) will return the favor. In fact, two monsters can be peacefully defeated through flexing: Temmie who thinks that muscles are gross, and Aaron, who eventually flexes himself out of the room after a heated flex-off with the player.
    • As for specific characters, there's the buff flex-happy Aaron, the beefy Royal Guards and especially the hot-blooded anime hero-wannabe Undyne, who at times during her battle will suplex a boulder for no reason other than because she can.
  • Recurring Riff: Several. Flowey's theme and the main theme in particular. There's also a Genocide route exclusive one used for 2 of that route's 3 exclusive bosses.
  • Recycled in Space: The game doesn't shy away from the fact that it's more or less MOTHER made for the modern era.
  • Red Oni, Blue Oni: The two skeleton brothers, Papyrus and Sans. Their clothes even indicate it. The former is a bit hyperactive and very passionate, the latter is laid back and lazy. Well, Sans acts that way, at least.
  • Rewarding Inactivity: In some rare cases, it's better to just stand still to avoid enemies' bullet patterns. Blue attacks enforce this: they won't hurt you as long as you don't move when they are passing. Orange attacks invert it: they won't hurt you as long as you are moving as they pass.
  • Rimshot: Two of the jokes told by Sans when you first meet him are punctuated this way.
  • Room Full of Crazy: In the demo, the manual after the Genocide ending. Almost every page is the same image of a black background with the game's title on it, and red text saying "That was fun. Let's finish the job."
  • Rousseau Was Right: According to Word of God, all of the monsters that the player encounters are basically good and only doing what they think is right, with the exception of Jerry. Even he seems to be mostly an unpleasant but not particularly destructive kind of evil.
  • Ruins for Ruins Sake: The aptly named Ruins, the first area in the game. It used to be where all the underground residents lived, but they expanded out over time, leaving only a few monsters scattered around.
  • Rule of Seven: The monsters need the power of seven human SOULs in order to break the barrier keeping them underground. They already have six of them when the player character stumbles into their kingdom...
  • Save Point: Appears as a yellow star-shaped icon. The manual describes it as a manifestation of your determination... something in the environment focuses the protagonist's will and enables them to record their progress.
  • The Scrappy: Jerry In-Universe: he's rude, ugly, smells bad and whines a lot, you "spare" him (and in turn the other monsters of him) by out-right ditching him.
  • Screw This, I'm Outta Here: While you can do this to any monster you're having difficulty befriending without affecting your Pacifist run, you have to do this to "defeat" Undyne peacefully... no amount of talking or stalling will allow you to spare her normally, you just have to flee the battle and keep running.
  • Sdrawkcab Name: Subtle. "Ebott" backwards is "Ttobe", which can be pronounced "Toby", as in Toby Fox.
  • Seasonal Baggage: The different areas of the Underground invokes imagery from different seasons.
    • The Ruins with its fallen orange leaves and ivy plants everywhere is quite autumnal. It is also the first area you are in after taking a fall.
    • Snowdin has — true to its name — snow everywhere. It is very obviously associated with winter.
    • Waterfall is bursting with plant life, and is a underground area full with water, otherwise know as a spring. Alternatively, New Home has flowers and characters that refer to the sun shining and the birds chirping, which can also remind one of spring, especially when considering Waterfall a "non-seasonal" zone for being a cave in a cave.
    • Hotland is very warm, and the students that live in the area are on summer vacation.
  • Second Place Is for Winners: In the Snail Race minigame, the best prize is obtained if you manage second place, on the logic that your snail mistakenly believes they won, and Napstablook (the one running the game) doesn't have the heart of correcting the poor animal out of their mistake.
  • Secret Test of Character: On a meta-level, Undertale as a whole seems to be this for the player. A major running theme is that your actions have lasting consequences even within a fictitious video game world. Even if you reset the game and try again, the game subtly (and not-so-subtly) "remembers" what you did the first time around: whether you went out of your way to spare a monster or whether you just killed it. The implication is that the first thing that the player decides to do reflects their true character, and if you go back and make a different choice after seeing the out come, you're either trying to hide your mistakes or just trying to see a different outcome for your own amusement.
  • Self-Deprecation: One save point in the Waterfall trash dump describes a very long process involving worthless garbage going down the falls into the abyss as filling you with determination. If you use that save point again, it just says, "Partaking in worthless garbage fills you with determination."
  • Self Fanservice: The game's fanart is absolutely infamous for how much of this it features.
  • Shall I Repeat That?: Early in the game, Papyrus gives you a set of rules to an absurdly complex tile puzzle, then at the end gives the player the option to hear the explanation again. Subverted in that Papyrus doesn't understand the rules either, and will initially mix up what tile does what in his second explanation before correcting himself. If the player then says that his second explanation confused them even further, he gives up, leaves the instructions on the ground and tells you to read through them on your own and do the puzzle at your own pace once you've figured them out.
  • Sheathe Your Sword: All battles can be won by "sparing" enemies instead of attacking them, though you may have to perform special actions first; a full Pacifist run requires this. This is practically its own puzzle during the first Boss Battle: the game starts telling you that talking won't work.
  • Short Distance Phone Call: If you call an onscreen NPC, you can see their mouth moving during their lines.
  • Significant Anagram: The title of the game itself is an anagram for "Delta Rune", the name of the Arc Symbol of the game. Also, the odd spelling of Asgore's surname ("Dreemurr") was probably chosen deliberately to be an anagram of "murderer".
  • Sliding Scale of Idealism vs. Cynicism: Depending on how you play, this could be one of the most light-hearted and idealistic games of all time, or one of the most cynical and horrific games of all time.
  • Slippy-Slidey Ice World: The Snowdin region, which is covered in snow and ice. It consists of a giant snowy forest and a cozy winter town. Yes, there's slippery ice as well, but it's used sparingly.
  • Solve the Soup Cans: Parodied with Sans' "puzzle", a word search on the ground. Literally nothing forces you to look at it, let alone complete it (it's also technically unsolvable because of a typo).
  • So Unfunny It's Funny: Papyrus, Sans and many other characters will often tell really bad jokes, which do end up being funny nevertheless.
  • Source Music:
    • If you stay at Snowed Inn, the neighbors next door snore to the tune of "Determination" (the song heard on the Game Over screen).
    • During Muffet's boss battle, the flavor text between turns occasionally mentions the spiders clapping and dancing to the rhythm of her theme.
  • Spikes of Doom: Early on the story, one puzzle is a field of spike panels. Toriel holds your hand through the right path out of concern, but if you go back afterwards, you'll realize the panels are harmless.
  • Splash of Color: A few areas, such as Toriel's and Asgore's bedrooms, are almost completely devoid of color... except for yellow flowers. The Pacifist ending reveals that the yellow flowers have quite a bit of plot relevance.
  • Squick: In-universe: if you decide to use Junk Food from Bratty and Catty to heal during the Mettaton fight instead of MTT-Brand food from Burgerpants (the former is a lot cheaper and heals enough to be worthwhile on a Pacifist run), you'll gross out the viewers and lose some ratings, which is a problem if you're trying to spare him.
  • Stage Whisper: If you try to back out of hanging out with Undyne when Papyrus has already entered her house, you'll hear Papyrus "shouting a whisper" for you to come back.
  • Star-Crossed Lovers: In Mettaton's opera parody, he sings about a forbidden love between him, a monster, and the player a human.
  • Stop Helping Me!: Toriel, during the Tutorial Level. Then later on, Alphys is very insistent on helping you. Both these instances end up making a lot of sense in-universe though, the more you find out about these characters.
  • Strength Equals Worthiness: The first boss will allow you to leave the Ruins only if you prove you're strong enough to defeat them. Deconstructed in that if you take the boss's HP down to zero, they really do die instead of just yielding the battle and recovering in the next cutscene as is typical for RPGs. You're fighting Toriel, who's seen many children die due to being too weak to survive and doesn't want to repeat this.
  • Stunned Silence: If you manage to lose the fight with the tutorial dummy by repeatedly missing your attacks, Toriel will be completely baffled, coupled with a Fascinating Eyebrow, before moving on like nothing happened.
  • Stupidity Is the Only Option: Before you encounter Muffet, the path through her lair is littered with spiderwebs that slow your progress before arriving in an area covered in one big web. It's obvious that you're going to get stuck if you keep going, but there's no other way to proceed.
  • Suddenly Voiced: In the entire game are two instances were an actual voice is heard. The first is during a Genocide run after your second encounter with Flowey instead of his usual creepy/maniacal laugh, the sound-bite used is a cartoony, high-pitched "That's a wonderful idea!". The second is during a Neutral/Pacifist run when you flip Mettaton's switch, which transforms him into Mettaton EX. After a flash of light, a sound-bite of him saying "Ohhh yesss!" in a deep, electronic voice is heard.
  • Suspiciously Specific Denial:
    • Toriel calls you to ask if you like butterscotch or cinnamon, then ask if you're okay with cinnamon/butterscotch (whichever you didn't choose), and finally to ask if you have any allergies. She denies she's up to anything.
    • Annoying Dog denying he ate one of Toriel's pies at the end of "Hard Mode".
    • On the Pacifist route, after the pretend date with Alphys, Papyrus sends her home from training early and wants you to find her at her lab for no apparent reason except he feels you should.
  • Sword and Sorcerer: The monster "Knight Knight" (a large hulking knight) tends to appear alongside the monster "Madjick" (a floating, grinning wizard).
  • Talking the Monster to Death: The standard way of "disposing" of enemies (and bosses) in a Pacifist route. Reversed in the Genocide route where Papyrus tries to do this to you.
  • There Are No Therapists: Many major character can be reasonably theorized to be mentally ill or otherwise dealing with serious emotional issues, especially Alphys, Sans and the Dreemurrs, but nobody ever mentions therapy. Granted, the game does take place over one or two days, so it's possible it's offscreen or the characters in question aren't taking the effort to seek help.
  • They Wasted a Perfectly Good Sandwich: Sans treats you to a a burger or fries at Grillby's and talks to you about Papyrus and a suspicious flower that whispers things to Papyrus. After that tense moment, you and Sans leave the counter without eating. An NPC remarks that the food is probably cold by now.
  • This Is Gonna Suck: As put by the intro to the final boss fight on the Genocide route: "You feel like you're going to have a bad time." You will, due to said boss being the hardest in the game.
  • Toe-Tapping Melody:
    • You can put on some ghost tunes at Napstablook's house - while Napsta loves the rhythms and vocalizations, if you leave afterward and encounter Aaron and Woshua, they won't bother fighting because the music leaves them too freaked-out, and the pair eventually flee. As a Brick Joke, Aaron will become a ghost inspector in the Pacifist Ending credits.
    • If you want to give Shyren lots of love, you can keep singing with her during her battle - by doing so, you start a band with her that gets popular, while Sans sells tickets and tosses toilet paper and socks at you. She's so catchy that the Golden Ending features her getting over her stage fright and performing with Napstablook and Mettaton. The hitch is that it's easy to die to her, because the more confident she gets, the more "musical note" bullets she will send your way. The scene is also much less funny if you killed Papyrus during that run.
  • Too Awesome to Use: The slice of cinnamon butterscotch pie Toriel gives you is specified to replenish all HP. When you're at a point where the cheapest foods will replenish most health, it'd be a shame to just waste this. Invoked, considering it has a special effect on Asgore at the other end of the game.
  • To Win Without Fighting: Refusing to kill monsters is necessary to obtain the Pacifist route (and the Golden Ending).
  • Tradesnark: A couple of NPCs in Hotland will share their favorite Mettaton Moment™ with the player.
  • A Tragedy of Impulsiveness: This is likely the ending a first-time player will get. See Wrong Genre Savvy below for more details.
    • The backstory: Asgore made the proclaim of actively hunt for human souls very shortly after the death of his children, while he was still hurt and grieving. By the time he actually got a human soul, he found himself unable to follow the quickest way to get more (namely, absorb the soul and get outside for them), as he actually was too nice to do it. This, on turn, caused Toriel to abandon him, as she saw this hesitation as cowardly. By the time the player character enters the Underground, Asgore is still chained to the words he said in impulse while mourning, unable to take them back because that would mean the loss of hope of his subjects, but dreading that another human falls as he doesn't actually want to commit more murders.
  • Train Problem: During his quiz show, Mettaton throws an "easy one" at you, to trip you up. Likely put there to make you notice (if you hadn't already) that Alphys spells out the letter of each answer to the quiz show with her hands.
  • Trauma Inn: Snowed Inn and the MTT Resort heal your HP beyond max if you stay in a room.
  • Tsundere:
    • Examining a cactus in Toriel's home will result in the observation, "Ah, the cactus. Truly the most tsundere of plants." Coming back at the end of the game, and this changes to "It's not like this cactus was waiting for you to come back or anything...".
    • The Tsunderplane enemy in Hotland is a tsundere airplane. Its in-battle actions are reminiscent of the typical tsundere behaviors in anime (like turning up its nose or "accidentally" bumping you with its wing). The way to peacefully defeating it, of course, is to get it all flustered by making a romantic advance towards it.
    • Undyne's behavior towards the player ends up being quite tsundere, should you attempt to befriend her. Though arguably, it's more of a different "dere".
  • Tutorial Failure: Flowey's tutorial parodies the trope; the player will likely find that they've learned nothing from it. Then Toriel comes in to pick up the slack; she's a bit more informative, though the player might be somewhat frustrated by all the hand-holding.
  • Unexpected Gameplay Change: The idea of having bullet-hell minigames in between your turns is already novel enough, but every so often, a new mechanic comes along to shake things up.
    • Papyrus' battle introduces gravity, and you have to jump over the bullet hell obstacles.
    • Undyne's battle uses a sort of Dance Dance Revolution-esque mechanic, where you have to stand in place and face the correct direction in time to block the arrows.
    • Muffet's battle restricts your movement to horizontal lanes, and you must switch to avoid oncoming obstacles, not unlike Audiosurf.
    • Mettaton's battles gives you the power to shoot through otherwise impossible-to-dodge obstacles.
    • Flowey's battle is the most dramatic departure from the norm; he dispenses with the RPG combat elements altogether, and it becomes pure frantic Bullet Hell.
  • Unexpected Shmup Level: Alphys can turn your soul yellow, turning it upside down and allowing it to shoot projectiles. This turns Mettaton fights into miniature shoot-em-ups.
  • Unreliable Narrator: The pre-sealing details given by the monsters clashes with the opening narration. The intro claims that the war between humans and monsters was an even-sided war with reasons lost to time that ended with humans victorious after a long battle, whereas the version given in Waterfall paints it as more of a massacre. The Waterfall story also says the barrier was erected by seven mages, but only one mage is seen making the barrier in the intro. Gerson claims the Delta Rune predates written history and symbolizes the monster's escape, but the intro shows the monsters wearing it prior to the sealing.
  • Unstable Equilibrium: As your LV increases, your attack stat increases and enemies' defenses passively decrease. Thus, as you gain XP and increase LV, you get stronger and stronger until you steamroll almost everything in your path, and nothing can stand up to you.
  • Unwinnable Joke Game: The word search that Sans lays out in hopes of stopping you (yet can not only be walked past, but still does nothing to stop you if you read it). Every word can be found... except for the gibberish word that looks like the top row of letters but is actually a letter off.
  • The Very Definitely Final Dungeon: In the Genocide and Neutral routes, the CORE is the final area that has random monster encounters before the Final Bosses; in the Pacifist ending, the True Labs are this instead.
  • Video Game Caring Potential:
    • To be caring, there's a lot more to it than just being "kind". In order to get the best ending, you have to work your ass off sparing monsters, being nice to everyone, and so on. This takes a lot of work, and one of the points that Flowey makes during the Pacifist run is that you may not be able to keep this up for long.
    • There are two specific instances of this trope that have nothing to do with achieving the Pacifist ending and in fact don't benefit you in any way. One is the snowman near Snowdin who asks you to take a piece of himself with you so he can see the world. Said piece of snow then takes up a valuable slot in your inventory for the entire game (though you can eat it to regain HP) and carrying it to the end of the game simply gets you a remark from Sans that you "made a snowman very happy." The second is giving an umbrella to - of all things - an inanimate statue caught in the rain, which activates a music box playing the Undertale theme, and will net you some lines of dialogue if you call Undyne next to it later on, as well as giving you a hint to a musical puzzle if you stand there for a minute as a small reward.
  • Voice Grunting: Even though the game does not have any genuine voice acting, every major character makes a different sound when their words are dispensed on the screen. The ability to recognize them by this voice alone is essential for a couple of twists. The only two characters who have genuine voiced dialogue are Flowey and Mettaton, but only one sentence each.
  • Waiting Puzzle: One option for getting past the force field in the CORE is to just stand in front of it for a few minutes. "I cannot fight. I cannot think. But with patience, I will make my way through."
  • Watsonian Versus Doylist: The game thoroughly explores the dilemma behind and between both of these stances and ultimately deconstructs the whole concept.
  • Well-Intentioned Extremist: The majority of monsters only want to kill you so that they can use your SOUL to escape the Underground.
  • Wham! Shot: Has a few that set the stage for the rest of the game:
    • If you feel forced to fight Toriel, you may take it a little too far. She collapses, gives a speech that she is proud that you are strong, and fades away. On a a genocide route, or if you backstab her, she'll die Laughing Mad saying that you will fit in just fine in the Underground. You can only avoid either option if you refuse to fight her, and convince her to let you go into the Underground. Toriel's Final First Hug when she lets you go peacefully isn't as dramatic as her death, but it has made a few players cry.
    • As you walk into Waterfall, a spear nearly hits you in one area. You pause in alarm, as a knight emerges, Lady Undyne! Time to run for it before she grabs your soul.
    • Some optional areas show monsters with eyeless faces that disappear as soon as you talk to them. You may even unlock a secret room with a figure named Gaster.
    • The first Neutral run ends with you defeating Asgore. Yet...you can still spare him. He offers that if he reconciles with his wife, they can all live together and be a family. Cue very familiar pellets surrounding and killing him. Flowey appears, giving an Evil Laugh as the souls surround him; he reveals that he took the time to grab the souls while you were fighting Asgore. He glitches out the game, having written the world by wiping everyone out.
  • When All Else Fails Go Right: The starting point is the westernmost point in the game, and you largely move towards the east, with the end of the game being the easternmost point.
  • Wrong Genre Savvy: In a way, the player may befall to this, specially when playing blind. Play in a way typical of a traditional gamer, ignoring the Mercy mechanics (and the game slogan), and you are for a nasty shock when you either get in the darker side of the Neutral run, with the characters hate you and berate you for your murderous ways, or you get locked in the Bad route, where you basically are treated as The Dreaded. The game is intended as a Deconstruction of JRPG mechanics and 100% Completion.
  • You Mean "Xmas": Snowdin has a tradition of giving presents under a tree, which originated when some pranksters started putting decorations on Gyftrot.
  • Your Size May Vary: Characters' battle sprites aren't drawn to the same scale relative to each other as their overworld ones. It's not too noticeable for most of the game, mostly because major characters tend not to appear in-battle with each other, but during the True Pacifist ending, all of the main characters are shown together both in the battle interface and on the overworld, and there are obvious differences. For example, Asgore is, judging from his overworld sprite, supposed to tower over everyone except Toriel, but in-battle Undyne is almost a full head taller than him. The thing is even muddied in official art.
  • Zero Effort Boss: Most bosses and enemies on a Genocide run can be killed before they even get to act.
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