Twister: I will?
A 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.
Expect things to go badly if the character doesn't know what to do with it.
- 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 an 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. (It's absent from the dub, but Kaiba calculated when he gave Yugi the Fiend's Sanctuary card that it would do nothing more than increasing 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. And as stated, Yugi did win, the card helping a great deal.)
- Zigzagged in the first season of Yu-Gi-Oh! GX. After Judai defeats Daitokuji-sensei, 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.
- In Catherine and Her Fate, Catherine appeals to her Fate, to soften her hardships. Her Fate responds by giving her — a skein of silk thread. Catherine nearly throws it out in annoyance.
- The second part of Slayers Trilogy starts out with The Lord of Nightmares tasking Lina with gathering the Wingless Ones, but she does not explain anything about it, merely telling "You will soon have help".
- 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.
- 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 Pirates of the Caribbean: Dead Man's Chest: "I gotta Jar o' Diiirt!" Subverted somewhat in that it really does turn out to be useless. The useful stuff was taken out beforehand.
- 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 Cliffhanger before it was actually used. Fortunately, one of the fan theories proves correct.
- Parodied in Reign of Fire. A mechanized column of heavily-armed Americans turns up, and their leader demands to speak to whoever's in charge.
Quinn: I'm going out. Anything happens, you know what to do.
Creedy: Uh no. I have no idea.
Quinn: Yeah, me neither.
- 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 hands 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. It ends up summoning Ozzy Osbourne.
- 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.
- Patton. "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." Presumably "throw up" was not the answer he was looking for.
- Superman II. While escaping from prison, Lex Luthor sends his Bumbling Sidekick out first. Luthor's minion asks what he's looking for and is told, "You'll know it when you find it." Cue minion bumbling along until he encounters a rope ladder. "I found it!"
- Captain America: Civil War. During the airport battle, Ant-Man tells Captain America, "On my signal, run like hell." Shortly afterwards he uses his abilities to grow to enormous size.
Cap: I guess that's the signal.
- Dragonslayer: Ulrich tells Galen to destroy his amulet — "and me along with it...you'll know the time." Then he disappears. Later, when Ulrich is snatched up by the flying dragon, Galen smashes the amulet. Ulrich explodes, taking the dragon with him.
- In Perry Moore's superhero novel Hero, the protagonist's mother gives him a thinly-disguised Kryptonite ring 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 by his estate's executor with cryptic hints. Each proves critical to their quest. Justified because Dumbledore is dead and since the aforementioned executor takes anything he deems "suspicious" he can't outright tell them. It does, however, raise the question why he didn't just give it to them earlier, transfigured if necessary, since he knew he was going to die almost a year in advance. But hey, it's Dumbledore.
- Subverted in Harry Potter and the Order of the Phoenix. At the beginning of the book, Sirius gives Harry a package with instructions to use it if Harry needs Sirius' help. Knowing that any attempt to help Harry by Sirius would bring with it an unacceptable risk of being captured and re-imprisoned, Harry resolves never to use it, promptly forgets about it, and doesn't consider that it could be equally useful should Sirius need Harry's help. It turns out to be an enchanted mirror allowing safe, easy two-way communication between the two. A lot of tragedy could have been avoided if Sirius had just told Harry what it was instead of being mysterious about it.
- 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. The clues end up saving his life in the climax, as they are detailed instructions on how to evade a band of savages that is attempting to capture him.
- 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. He doesn't.
- 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 everyone's problems. He actually ends up subverting his own lampshade by being an unusually bright and logical protagonist, and it's 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.
- In Lord of the Rings, Galadriel gives Frodo a phial of water from her fountain, saying "May it be a light to you in dark places, when all other lights go out". While this has an obvious literal meaning, it takes on a new significance when Sam is fighting Shelob and it turns out that the light from a Silmaril is a more potent weapon against the daughter of Ungoliant than the sharpest of swords could ever be.
- In Lovely Assistant (by Geoph Essex), when Jenny finally gets the chance to take a lesson in Grim Reaping from Caravel, Caravel essentially says "You Will Know What to Do" when it comes to using the sword and guiding people to the afterlife. As it turns out, it's literally true: Jenny instinctively knows what to do when she gets there and is pretty surprised by that fact herself. (The implication that the experience is better than drugs or sex for Grim Reapers suggests that it's just a "natural" thing for them to do, like fish swimming or birds flying.)
- Near the end of the book, Caravel invokes this trope verbatim when Jenny is leaving for her appointment with the Ice Cream Psycho.
- In The Power of Five: Evil Star, the Incas give Richard a Tumi (sacrificial knife), warning him that he will regret ever having been given it. In the final book Oblivion, Richard uses the knife to kill Matt, sparing him from further pain and torture, and allowing past Matt to be summoned to the present.
- The Dresden Files: This pops up occasionally when dealing with the Knights of the Cross, men and women who are Chosen to wield a holy blade with a Nail from Jesus' Crucifixion in its hilt. In the fifth book Shiro, wielder of Fidelacchius the Sword of Faith, bequeaths the Sword to Harry before trading spots with Harry to save his life from his old nemesis Nicodemus Archleon, a millennium old man who works with a Fallen Angel tasking him to be the Sword's Custodian. Shiro tells Harry that he will know who is the right person to give the Sword to and when it should happen. Five years later Harry is trying to save a young girl and local crime boss from Nicodemus. During a parley, when what Harry has to offer isn't enough for Nicodemus to agree with the trade, Harry has a realization that this is the moment Shiro spoke of. Nicodemus is the man Harry needs to offer the Sword to. Harry adds to the pot Fidelacchius and plays on Nicodemus' buried fears that God will somehow stop his plans once again. This is enough for Nicodemus to agree to the trade. The end result is Fidelacchius remains with the good guys, the girl and the crime boss are saved, and Nicodemus is nearly killed by Harry.
- The trope ends up subverted when Harry trusts Karrin Murphy with Fidelacchius, only for it to end up broken when she tries to use it for unjust ends. Harry believes he failed, right up until Waldo Butters, Jewish coroner extraordinare, takes up the shattered hilt of the sword and uses his faith in other people to recreate the sword as a lightsaber. Harry didn't choose Butters, but the sword did, and the next book (Peace Talks) implies that God and/or the Angels let the sword be destroyed so that it could potentially come back even stronger.
- Rhythm of War (fourth book of The Stormlight Archive): Mraize tells Shallan she'll know what to do when she finds the person he's told her to locate. When she does, and discovers he's one of the immortal Heralds, she is shocked and rushes to call Mraize and complain that she had absolutely no idea what to do. He says rather smugly that she did exactly what he wanted: contact him for further instructions.
- 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.)
- In the Supernatural episode "Appointment in Samarra", Death instructs Dean to keep researching the souls and says that he will understand when he needs to. To his exasperation, Death ends up having to spell it out for Dean anyway.
- Person of Interest. The Machine works entirely like this. All it gives to people are the social security numbers of a person of interest. It is up to the people to research that person and understand who they are, how they might be in danger or be the one to cause harm, whether locally or on a terrorist-level of danger. While its human agent Root takes this entirely in stride, the rest of Team Machine find it rather frustrating.
- In Ratchet & Clank Future: Tools of Destruction, after a slight Fourth Wall breaking moment, The Plumber gives the protagonists a "3 3/4 Centicubit Hexagonal Washer" claiming it will "come in handy". Sure enough it helps them escape from a collapsing alternate dimension at the end of the game.
- 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 by giving them to himself in the past, as this is nothing but a puzzle whose solution is to give yourself the items in the exact order you received them in earlier. Deviating from this causes a time vortex to open up and take you to the time before you met yourself. One of the more amusing ways to screw up this puzzle is by using the gun on your counterpart...
Guybrush: "Whoa! I guess it's true that gun owners are nine times more likely to shoot themselves."
- 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 legendary one with a badass name, is not readily apparent.
- Parodied by Maddox in his review of The Matrix Revolutions, which he hated. He particularly criticizes the Cryptic Conversations between Neo and the Oracle.
Neo: What do I need to do?
Oracle: You KNOW what you need to do.
Neo: Where do I need to go?
Oracle: You KNOW where you need to go.
Neo: Who do I need to see?
Oracle: You KNOW who you need to see.
Neo: What do you want?
Oracle: The same thing you want.
Neo: How will I know I'm making the right choice?
Oracle: You will KNOW...!
- Parodied in the Family Guy episode "The Cleveland-Loretta Quagmire", 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 Cleveland. It bounces off harmlessly, falls to the ground, and both parties look expectantly at it (although Cleveland was probably looking at it less expectantly and more "Did he just throw a banana at me?"). Nothing happens, because Quagmire was too Genre Blind to do a Banana Peel gag, and after a moment, the chase resumes.
- 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."
Dojo: I know. This kinda stuff drives me crazy too.
- Episode 52 of Scooby-Doo! Mystery Incorporated: The Annunuki, in the host body of Nova (Brad and Judy's Cocker Spaniel), tells Scooby this during the final battle with the Nibiru Entity. Scooby gets very frustrated and tells her to just tell him what to do since they are in such danger.
- In the episode of The Smurfs "The Cursed Country", Hefty and Smurfette have to lead Peewit and Sir Johan to their village, which is being held hostage by a wizard and his trained dragon. The wizard Homnibus lends them a stick that can produce water should they need it; however, the first time they try to use it, they discover it produces salt water, making it unfit to drink. Later, however, after waiting for the dragon to fall asleep, they realize it's true purpose: Johan cuts a small sliver of it and gives it to Papa Smurf, who is then able to crawl into the beast's belly and use it there, rendering the dragon unable to breathe fire and nearly powerless.