Amnesia Danger

The main character or characters has amnesia in a situation which would be trivially easy to handle if they could remember how to deal with it, but is extremely dangerous because they can't.

Often a sign of writers desperate for plot ideas -- not always, because there have been some good stories on these lines, but usually. It's especially common in sci-fi and fantasy shows, because they have plenty of Applied Phlebotinum lying around to explain the fact that the amnesia's so precise. Compare Drama Preserving Handicap, contrast Forgot About His Powers.

Advertising

 * A mid-80's Cocoa Puffs ad campaign guest-starred Popeye with the cereal's mascot Sonny the Cuckoo Bird. Popeye is trying to help Sonny escape the kids who constantly amuse themselves by tempting him to cave in to his Cocoa Puffs addiction. Popeye's ship runs aground and he gets hit on the head, forgetting all about spinach! Kids were asked to write in to remind Popeye of his secret weapon. Naturally, the sailor eventually remembers, but even spinach isn't strong enough to fight the addictive power of Cocoa Puffs!

Anime and Manga

 * It happened to Duke Fleed in one of the manga versions of UFO Robo Grendizer -one of the Mazinger Z sequels-. In the first episode of the Gosaku Ota manga, Duke does not remember anything about his past since Dr. Umon had wiped his memory's out so he could live peacefully without remembering anything about his destroyed home and slaughtered family. When the Vegans began their attacks the shock brought his memories back and he remembered he could, you know, put a stop to that.
 * It also happened in the sequel to Cutey Honey, another Go Nagai creation. In the first episode of New Cutey Honey, she spends a long while running from the Mooks of the Big Bad until she gets her memories back and remembers she can kick their sorry butts easily.
 * Kaito in Mermaid Melody Pichi Pichi Pitch could have saved both Lucia and Michal a lot of heartache and jealousy if he just remembered who he was and which one was his girlfriend.
 * Happened to Lelouch in Episode 1 of Code Geass R2. He could have dealt with Britannia's attack with the help of La Resistance if he hadn't had his memories wiped, but instead of kicking Britannia's ass he spent the latter half of the episode running around. Uselessly.
 * He more than makes up for it when he gets his memories back by
 * Inverted with Yue in Mahou Sensei Negima's "Magical World" arc: Her amnesia results in her being admitted to a prestigious school of magic and later to a group of magical knights, completely shielding her from anyone attempting to claim the huge bounty on her despite said knights serving as law enforcement personnel. Asuna Kagurazaka may turn out to be a straight, if unusually long-term, case: Her missing memories have so far been shown to include her own magical-combat abilities and her time being escorted by the Ala Rubra, and probably include more knowledge of Negi's father than anyone else has yet shared with him, all of which would be helpful to have as she, Negi, and the gang are on a quest to find Negi's father.
 * The Stand Jail House Rock in the "Stone Ocean" part of Jo Jos Bizarre Adventure takes advantage of this to an extreme degree. The Stand attenuates the victim's short-term memory to the point that they cannot retain more than three facts at any time. If the villain for instance shoots four bullets at Jolyne, at the point she notices the fourth one she'll forget they fired the first one.
 * Stop and think for a second about how much less story there would have been in Rave Master if Elie had never lost her memories.

Comics

 * In a Black Panther Arc Photon, once leader of The Avengers, had kind of forgotten who she was and how to use her powers. This was unfortunate as the city she in was having a vampire apocalypse, a situation her light based powers were ideal for dealing with. What's worse is that most of the other heroes don't really know her or what she's capable of and therefore aren't trying to get her to remember.
 * Matter of fact, by the end of the story she still doesn't have her memories back (despite being a little better with her powers). Somewhere down the line she eventually does lose her amnesia though.
 * Ms. Marvel's origin involved the character not knowing she was a superhero and having blackout spells whenever the titular character would go out and do some superheroin', but was "just an ordinary woman" when she wasn't. Similar to Harry and Norman Osborn in the Spider-Man comics not knowing they were Green Goblin from time to time.
 * Rose Forrest/The Thorn in DC Comics' Rose And Thorn had the same deal. In current continuity, Rhosyn Forrest knows about Thorn, but doesn't trust her, and certainly can't do the things she does.
 * In one issue of the Batman comics, Bruce forgot that he was Batman. Nightwing and Robin had to team up to protect him and get him to remember.
 * In the Don Rosa comic "Forget It!", Magica de Spell puts a curse on Scrooge and Donald Duck, which has the following effect: whenever they hear their names spoken in conjunction with another thing, they immediately forget the thing in question ever existed. Thus, they can't even open a door, because she makes them forget what doors are. And this is just the beginning...
 * In one Jonah Hex story, Jonah is buried by a rockslide while dressed in the clothes of a preacher. Digging himself out, he cannot remember who he is. Finding a letter addressed to the preacher in his pocket, he assumes that he is the preacher and takes the preacher's new posting as an army chaplain.

Films

 * In Memento, the main character, having reached the hotel room of the man he is seeking, hides in the bathroom and prepares to ambush his quarry with a wine bottle. However, as he cannot hold onto any new memories for more than 20 minutes, he forgets why he is there and assumes he was about take a shower, leaving him unprepared for when the enemy does arrive.
 * This happens more than once. At one point, the main character assumes he's chasing a man until said man starts shooting at him, at which point he realizes that the man is chasing him. In fact, the guy chasing the main character with the gun is the same guy the main character's preparing to attack with the wine bottle.
 * Men in Black II, where MIB needs the memory-wiped Kay to come out of retirement to deal with the crisis at hand. And when he remembers being in MIB, it turns out he wiped his memory of the critical event, so they're still in trouble.
 * In the Dana Carvey film Clean Slate, the protagonist is a detective suffering from amnesia who's supposed to testify in court but can't remember the details of the case he was working on.
 * Star Trek Insurrection's plot is kick-started after Data is damaged in a way where he retained no conscious memory, and was driven only by "moral subroutines."

Literature

 * In the fourth Artemis Fowl book,
 * In Animorphs Megamorphs #1: The Andalite's Gift, Rachel endures head trauma and loses her memory. As a result, she spends most of the book lost in the woods since she can't remember who she is or that she can morph to help find her way out.

Live Action TV

 * Buffy the Vampire Slayer, "Tabula Rasa" - there are elements of this in "Halloween", too.
 * Stargate Atlantis, "Tabula Rasa"
 * The long Lois amnesia plot and the Superman amnesia story in Lois and Clark (the latter of which was a remake of the Superman amnesia story in The Adventures of Superman).
 * Played with in Dollhouse episode 4, "Gray Hour". The whole theme of the series revolves around certain characters which are given amnesia and given new personalities almost twice an episode. Then halfway through this episode,  From there this trope is played straight.
 * Appears several times in Star Trek the Next Generation. In "Conundrum", the entire crew of the Enterprise have their memories erased by aliens, in order to trick them into attacking the aliens' enemies. In "Thine Own Self", Data wanders into a village on a primitive planet, with no knowledge of who or what he is, bearing a case full of radioactive metal fragments; not knowing the danger, he sells the fragments to the local blacksmith.
 * There's also the TNG episode "Clues" in which the crew of the Enterprise discovers evidence that their memories and records have been tampered with and backtracks...only to discover that one of the things they've forgotten is that they had agreed to having their minds wiped in the first place.
 * Charmed: Prue becomes a teenager and loses her memories in "Be Careful What You Witch For", getting her in trouble with an evil dragon warlock. Piper loses her memories in "Valhalley of the Dolls", causing explosions galore. Leo loses his memories in "The Seven Year Witch", making it almost impossible to make his way home and return to Piper and his sons.
 * Used in an episode of My Hero in which Thermoman, whose powers include erasing people's memories, inadvertently erases his own memory of being Thermoman. This, of course, conincides with the discovery of a giant meteor hurtling towards the Earth.
 * Occurs in at least three MacGyver episodes, where Mac loses his memory due to being drugged, shot in the head, etc.
 * Phelps loses his memory in the middle of a mission in Mission Impossible, when he's supposed to walk into a trap in order to snare the villain; to complicate things, the person who tries to help him assumes he's the person he had been pretending to be.
 * Shannon loses her memory too in the second series. To complicate things further, the guy she was supposed to fool proposes to her...
 * Inverted in Honey I Shrunk the Kids, after losing her memories of everything that happened after she was 16, Diane is able to easily win a court case that she couldn't identify with because she was a grown-up.
 * Part of the central premise of Quantum Leap: Sam Beckett was an absurdly intelligent polymath who bordered on being a God Mode Sue, but the effect that time travel is constantly having on his memory prevents him from figuring out a way to get back home.
 * On Angel
 * In another episode, the heroes regressed back to their teenaged selves. Most of them had little or no fighting experience, and so the team found themselves unusually vulnerable - in addition to wrongly figuring out what was going on.
 * Happens a lot on Fringe with the character Walter Bishop, who can't remember science experiments he conducted before entering a mental hospital. Often this information is vital to the case they are trying to solve.
 * In early episodes of Heroes, the Niki/Jessica relationship somewhat similarly to Rose and Thorn from the Comics folder. Niki would wake up from a blackout and see a bunch of corpses strewn about, no idea how they got there.
 * In the Doctor Who two parter "Human Nature"/"Family of Blood", the Doctor turns himself human and puts his Time Lord memories into a watch in order to hide from the Monster of the Week. He intends for Martha to open the watch and give him his memories back when the aliens show up, but it gets stolen, leaving them all in danger.
 * This is the main threat of
 * In the first Kamen Rider Den-O movie, the shock of meeting his past self causes Ryotaro to develop amnesia of everything relating to the Imagin and his being Den-O, which means that for much of the movie they don't have access to Den-O's powers.

Video Games

 * Used in Phoenix Wright Ace Attorney: Justice for All as an excuse to give a tutorial to players that never played the original game. In truth, Phoenix has all but solved the case when he gets clonked on the head and loses his memory.
 * Considering the sheer amounts of experience the protagonist from Planescape Torment would have had were it not for his amnesia, the entire game could be considered an example of the trope, especially seeing that
 * This is so common in Interactive Fiction in general as to be a Discredited Trope.
 * The short IF story 9:05 is a particularly nice example.
 * Used in Breakdown, a game for Namco on the original Xbox, to great effect. It's also a subversion in that you're undergoing a cure for your amnesia by playing the game. The wacked out hallucinations, it turns out, are false memories. The part in the beginning where you're told that you promised to save someone then makes sense, as on the way back to an earlier point in the plot, you promise to save them. An earlier comment made by a scientist saying that she knew too much for someone who doesn't work there is explained when she doesn't know who you are and decides to help you save the Earth (by going back to an earlier part.) And the part where the penultimate boss asks "Motivation: Inconclusive. Derrick? Do you feel any different?" after he kidnaps the girl takes a new meaning (if it wasn't already the PUB's meaning)
 * In Kagetsu Tohya, the Tsukihime sequel, Shiki has been made to forget about what his eyes can do . So he repeatedly comes upon circumstances that would be no big deal and then wonders why he thought he could deal with such a thing. Like being chained or locked out of a room.
 * Nethack. Just Nethack. Amnesia (Mind flayer, or reading a scroll of amnesia) attacks can erase parts of the map, and, even worse, will unidentify some of the items you have formally identified (Whait, was the purple or the black potion the one of extra healing? I'll try the black! *fall asleep while under attack*

Web Comics

 * In the Sluggy Freelance story arc "Oceans Unmoving" . Made a delicious flavor of ironic by the fact that he gave himself amnesia.
 * Freefall: Florence gets into this.
 * Bug subverts this trope (and all for the doctor's amusement).

Web Original

 * In Wormtooth Nation, main character Sam kicks off the very first episode this way. His partner Dimitri abandons him after he's been "nixed" by "wormtooth gas" since he's now useless to the cause.

Western Animation

 * A Bottle Episode of Sealab 2021 has the crew trapped in a supply closet by a malfunctioning door. The only person who knows how to fix it suffers an amnesia-inducing blow to the head, then keeps getting knocked out.
 * Danny goes through a unique form of memory loss in Danny Phantom that also resulted in power loss. Very bad timing when there's a ghost getting more and more powerful by the minute.
 * Subverted in Kim Possible: when she loses her memory, all she needs is someone to tell her she can kick ass and she does.
 * The Pixar film Finding Nemo has Dory, a fish with short-term memory loss. This is also one of the most realistic depictions of amnesia in fiction.
 * An episode of The Batman has Bruce Wayne forget all about being Batman. While it doesn't take much for Alfred to remind him who he is, he still doesn't remember any of his training.
 * There was an episode of Batman the Animated Series where Bruce went undercover to investigate why homeless people were being kidnapped, and was himself kidnapped and ended up with amnesia. He was forced to work in a mine with all the other kidnap victims until he remembered who he was.
 * Jimmy Neutron goes through something like this in one of the early episodes when he "drains his brain" to make himself less hated in class. Of course, he drains it to the point where he is nothing more than a drooling idiot, just in time for a giant meteor to be found hurling toward their town. Of course, the meteor  in the first place.