You Will Know What to Do

"Reggie: Take this walkie talkie and hide. You'll know what to do. Twister: I will?"

- Rocket Power

Stock phrase when the hero is presented with a Plot Coupon.

Played for laughs when the item in question May Help You On Your Quest.

Usually translates as "We're not going to tell you what the magic amulet does yet in order to keep the audience in suspense," or, more cynically, "We don't actually want to do the work to prevent a Deus Ex Machina ending, so we're throwing this in now to make it look like we had it planned all along." Sometimes, in a very long arc, it can also mean "We haven't worked the resolution out either, so we're tossing in a random item now. The author will work out what it's for later." Perhaps they know that an unspoken plan always works?

The general gist of things is that the item will go forgotten for the rest of the story, and then, at a key moment, when all else has failed, he'll suddenly realize what the Plot Coupon is for, pull it out, and save the day.

Can refer to a character's generally-useless special ability instead of an actual physical object.

Often parodied these days by having the coupon itself be something not merely innocuous, but outright ridiculous.

An offshoot of Chekhov's Gun and often used as ammo for Schrödinger's Gun. Related to Figure It Out Yourself, and occasionally answered with I Just Knew.

It has never been revealed what would have happened if they didn't know what to do with it...

Anime

 * Justified/Handwaved in Rave Master. Haru Glory, wielder of the Ten Commandments sword, initially can only create the "Explosion Sword." Eventually, he is given the Rave of Wisdom, which gives him knowledge which is only gradually unlocked and told "You Will Know What to Do." From then on, he's able to whip out new forms of his sword as the plot demands. This comes with understanding of how the powers work, so he can from then on also call them forth at will.
 * This was sort of the case in Yu-Gi-Oh! where Kaiba gave Yugi his copy of Fiend's Sanctuary before the final battle with Marik in Battle City. Kaiba knew it was an effective weapon against the Winged Dragon of Ra, but he just told Yugi to take it and nothing else, figuring that if Yugi was capable of winning at all, he could figure out how to use it on his own. Fortunately, he did.
 * Although, while it is absent from the dub, Kaiba calculated when he gave Yugi the Fiend's Sanctuary card that it would do nothing more than increase Yugi's chances against Marik from 3% to 20%; he still doubted Yugi stood much of a chance. Kaiba told Ishizu that he only helped Yugi to prove her wrong about the bond between them when he lost. At least that's what he claimed his motive was. And as stated, Yugi did win, the card helping a great deal.
 * Zigzagged in the first season of Yu-Gi-Oh! GX. After Daitokuji-sensei loses to Judai, he gives Judai the Emerald Tablet before his physical body dies. Later, before he duels  Kagemaru, Judai finds a card called Sabatiel - The Philosopher's Stone in the book, and figures he should add it to his deck. Just as he's at the end of his rope, he draws the card, but still has no idea how to use it. Daitokuji's spirit appears and is about to tell him exactly how to use it, but then  he's swallowed by his cat Pharaoh before he can tell Judai the complete effect. Fortunately, Judai is able to figure that out on his own.

Comic Books

 * Comic book example: In Exiles, when Beak joins the team, they're told that he has "an important task ahead of him" and that "several worlds will hang in the balance", essentially making the character into the Plot Coupon.

Fan Works
""When faith calls to faith," she added, "you will know what to do with this.""
 * Drunkard's Walk: The trope name is said said word-for-word to Doug Sangnoir by Belldandy when she gives him a vial containing a drop of blood from each of the gods living at Tarikihogonji temple:

Film
"- I'm going out. Anything happens, you know what to do. - Er... no. - Yeah, me neither."
 * The main character of the movie Paycheck gets twenty of these in a Note to Self:, but this is more justified than most examples of this trope, as well as more skillfully executed. (It's pulled off with even more flair in the original short story.)
 * In the second Pirates of the Caribbean film: "I gotta Jar o' Diiirt!" Subverted somewhat in that it
 * The Autobox Matrix of Ass-Kicking in Transformers: The Movie. Nicely handled when, halfway through the movie, someone tries to use it and can't - it's not bad enough for the heroes yet.
 * In Galaxy Quest, the protagonists have to admit they don't know how the Omega-13 device works, because the TV series ended on a Cliff Hanger before it was actually used. Fortunately, one of the fan theories proves correct.
 * Parodied in Reign of Fire :


 * Done rather poorly in Troll 2, when Grandpa Seth hands Joshua a double-decker baloney sandwich, saying "take this, and only use it when you really need it". The last 5 minutes of the film is not a good time to introduce a vital Chekhov's Gun.
 * In the move Little Nicky, a group of angels hand Nicky a white orb before his confrontation with his brother, relaying a message from the God that when the times comes, he'll know what to do..
 * Happens in Starship Troopers. A literal Chekhov's Gun is handed out when Rasczak shoots and kills a communication officer that has been captured by a flying bug and then exclaims, "I expect any one of you to do the same for me." Later, Rasczak's legs are torn off by a tunneling bug, and he looks at Rico and says, "Rico... you know what to do!" Rico replies, "Yes sir!", and after a moment's hesitation, shoots him.
 * "When you put your hand into a bunch of goo that a moment before was your best friend's face, you'll know what to do."

Literature

 * In Perry Moore's superhero novel Hero, the protagonist's mother gives him with this instruction. Why she doesn't use it herself is less than clear.
 * Harry Potter and The Deathly Hallows: Each child gets a pleasant but seemingly useless gift from Dumbledore with cryptic hints. Each proves critical to their quest. Justified because   he can't outright tell them what they're for.
 * It does however raise the question why he didn't just give it to them earlier, transfigured if necessary, since . But hey, it's Dumbledore.
 * In John Moore's Bad Prince Charlie the titular prince is given a charm and told 'you'll know what to do when the time comes'. Given what kind of books Moore writes, when the time comes to actually use it as a last resort in the face of an enemy army they mock him by reciting the same phrase back at him.
 * In David Mitchell's Cloud Atlas, the character Zach'ry in the section "Sloosha's Crossin' an' Ev'rythin' After" is given a series of mystical and seemingly nonsensical clues by a fortune teller, explaining only that he will know what they mean when the time comes.
 * Metro 2033: Artyom is sent out by the Brahmins (who he's never met) to the Lenin Library (where he's never been) to find a lost book of prophecies (which he's never seen). They totally guarantee that he'll know where it is, though.
 * Done several times in the Sword of Truth. Prophecies given 3000+ years ago can be pretty nonspecific, you know? Additionally, the irrational use of this trope is lampshaded by Richard himself, who, as Seeker, is somehow expected to know how to solve everyones' problems. He actually ends up subverting his own lampshade by being an unusually bright and logical protagonist, and its revealed that one of the primary characteristics of a true Seeker is that they have this trope or the ability to invoke it before they get the sword.

Live Action TV

 * The amulet from the final episode of Buffy the Vampire Slayer.
 * Prison Break: Linc (when he and Michael go to break LJ out) tells his son "On the third, look out for otis right." and when LJ goes "huh?" Linc answers with the You Will Know What To Do.
 * Merlin had the Fisher King giving Merlin a waterglobe artifact in season 3 and telling him "When all seems lost, this will show you the way." But Merlin himself had to figure out what to do with it (and did so on accident,dropping and smashing it so the water fell into the bucket and Freya appeared.)

Mythology

 * Older Than Feudalism: The Bible is full of this trope. For example, God directs Abraham to take his son to the wilderness and offer him "in a place I will show you." When he gets there, he finds a sheep waiting to be sacrificed instead.

Video Games

 * In Ratchet and Clank Future, after a slight Fourth Wall breaking moment, recurring character "The Plumber" gives the protagonists a "3 3/4 Centicubit Hexagonal Washer" claiming it will "come in handy". Sure enough
 * Inverted in Escape from Monkey Island, Guybrush meets...himself from the future! When he does, the future self gives him a few things and says that he'll know what to do with them - only unlike all the other cheap knickknacks that only obtusely solve particular puzzles, he's actually given useful things, like a gun and a coil of rope. Guybrush then later uses these items


 * In Eric the Unready, you pull the legendary banana "Excalibanana" out of a stone, thus proving that you are the hero chosen to save the princess. You're then instructed to throw it in the pond, so you can magically summon it forth in its time of need. Exactly what use you'll have of a banana, even a legenday one with a badass name, is not readily apparent.
 * In the "Fate" story path of Fate/stay night, Archer takes time to tell Shiro that he is a "maker" not a "fighter", and that this will save him. Of course, Archer is.

Western Animation

 * Parodied in Family Guy, where Quagmire is told this and given... a banana. Further subverted when Quagmire is running for his life, recalls the cryptic advice, and throws the banana at his attacker. It bounces off harmlessly, falls to the ground, and both parties look expectantly at it. Nothing happens, and after a moment, the chase resumes.
 * Of course nothing happens, because Quagmire was too Genre Blind to do a banana peel gag.  was probably looking at it less expectantly and more "Did he just throw a banana at me?"
 * In one episode of Xiaolin Showdown, Omi is given the puzzle box that is the key to resealing Wu Ya, but isn't told how it works, save that, "it will open when the person who needs to open it opens it."