Because life just isn't as fun when you can see everything coming, there are several instances in fiction where the writers have chosen, for one reason or another, to give The Hero or the Big Bad an incredibly powerful, life-altering, plot-resolving super weapon (or super power), but not reveal what it is to the audience. They may only casually allude to it as "my secret weapon", they may pretend to "show" it but have it largely obscured by shadows or other objects in the way, or they may not even mention it at all until the pivotal moment. Regardless, this trope serves to create dramatic tension and keep the audience glued to their seats until The Reveal.
Something of a Discredited Trope, simply because it was overused to the point of nausea in the past, so it is more often Parodied or Played for Laughs in modern fiction. Also, while secret weapons used to be almost exclusive to super villains, more and more works are giving heroes a chance to invoke this trope as well.
This can occasionally overlap with Chekhov's Gun. For example, if it turns out that ballpoint pen the Big Bad is constantly twirling in his fingers is actually a world-ending nuclear device, both tropes would apply. If constructed poorly, this trope can also lead to Deus ex Machina.
See also It May Help You on Your Quest, for obscure items that come into play later, Forbidden Chekhov's Gun, where the characters must do something that they have been explicitly forbidden from doing, and Forgotten Superweapon, where the characters have a super powerful weapon, they just forgot about it. Stupid Jetpack Hitler and Soviet Superscience can often overlap with this trope, given that many of the things that both the Nazis and Soviets have invested and researched heavily into was the development of revolutionary weapons technology, of which many were kept top secret until much later on.
- In the first episode of Happy Heroes, Big M. asks Little M. to bring out his secret weapon. Little M. gives him a toilet plunger since a toilet floating among the space junk is what immobilized their ship, but then Big M. clarifies that's not what he wanted. Little M. then brings out the Jixie Stones and reads the directions to figure out how to use them as ammo for their ship... unfortunately, they don't make very good ammo and barely cause any damage.
- In Pleasant Goat and Big Big Wolf: Joys of Seasons episode 35, Weslie uses a soda can as a "secret weapon" against Wolffy while Paddi is watching, and later he uses a straw to shoot a poisonous cactus spike at Wolffy, again as a "secret weapon" and while Sparky is nearby. Paddi and Sparky sneak into Weslie's house at night to find the soda can and straw they think are Weslie's secret weapons, only for them to fail to work when Wolffy appears the next day and captures them both.
- All-Star Superman: Parodied when Bizarro Flash has a "secret weapon". It's a match. Furthermore, it gets rendered totally useless quite soon after he reveals it. Zibarro blew it out by mistake.
- Blake and Mortimer: The Swordfish plane in the first adventure "The Secret of the Swordfish". This trope is played completely straight; the weapon is referenced over and over in the story but never shown until the final pages.
- Judge Dredd: In the opening missile exchange of the Apocalypse War, East Meg One nullifies the Mega-City One nukes sent their way by unleashing a secret Soviet superweapon, the Apocalypse Warp. It erects a Force Field around their Mega City that sends all the enemy missiles to an alternate dimension, where they promptly blow up a hippie, peaceloving alternate Earth.
- Asterix and the Secret Weapon which is female legionaries, to take advantage of the fact that Gauls Would Not Hit a Girl.
- Asterix in Britain. A Roman officer sees Obelix lugging a barrel of the magic potion and demands to know what's in it (he's hoping it's Gaulish wine). He's told it's warm beer and loses interest. Later a messenger arrives telling him to be on the alert for two Gauls smuggling a secret weapon to the British.
Officer: [Face Palm] Warm beer!Messenger: No, that weapon's no secret. This is some kind of magic potion.
- Godzilla vs. Kong: Walter Simmons hints at Mechagodzilla's existence to the world on national television after Godzilla's Pensacola attack, without revealing precisely what it is (or that it's actually the cause of Godzilla's attack), and Team Godzilla's plot for most of the movie focuses on working out what Apex is hiding, culminating in Mechagodzilla's dramatic debut. It turns out that Apex's plan is to lure an aggravated Godzilla to Hong Kong with Mechagodzilla's signal, then set the Mecha on him to kill and replace him in one monstrous False Flag Operation so that they'll be hailed by the world as heroes.
- James Bond: Secret weapons play a large role in several of the movies, including:
- There is no shortage of this trope in the Star Wars films:
- The plans for one of these, the Death Star, serve as the MacGuffin for A New Hope. Luke stumbles across a droid carrying them, and must rescue Princess Leia so she can take them to the Rebel Alliance in hopes of finding a way to destroy the Death Star.
- Return of the Jedi has the Rebels learn of a second Death Star being built, and the plot centers around them going forth to destroy it. The Emperor is using it as bait to draw the Rebel fleet out into an ambush.
- In Attack of the Clones, the clone army serves as one for the Republic, so secret that even the Republic didn't know about it until the Army was delivered to them. In Revenge of the Sith, they are revealed to be a secret weapon for Emperor Palpatine as well.
- The early plans to the Death Star can be seen in Attack of the Clones, and the Death Star is shown in construction at the end of Revenge of the Sith.
- Rogue One centers on the Rebel Alliance learning of the Death Star's existence, and their efforts to find the engineer responsible for its development so they can execute him and securing the plans, which pivots directly into the plot of the first film.
- Harry Potter:
- Voldemort actually has more than one, including The Horcruxes and the prophecy he had been searching for.
- In the fifth book, Umbridge is so convinced that Dumbledore is plotting some kind of wizard uprising against the Ministry that Hermione is able to convince her that Dumbledore has a "secret weapon" hidden in the Forbidden Forest. Naturally, there is no such weapon, but the threat of it lures Umbridge out of Hogwarts and gets her captured by angry centaurs.
- In Dragon Bones, Oreg says he has been treated as this by many of his previous owners. He claims to not mind, as he doesn't like people, anyway, and is content being invisible. However, when he accompanies Ward on an adventure, pretending to be a bastard brother of Ward, he seems quite happy to be able to interact with people.
- Chapterhouse: Dune: The Honored Matres have The Weapon, which kills hundreds of troops instantly without blood
- HMS Polychrest, an Alleged Ship in the Aubrey-Maturin series, was originally designed to fire a giant rocket. The new weapon system failed, and Polychrest was converted into a conventional (albeit poorly-performing) sloop.
- In Daniel Polansky's Low Town, the Dren are a people who are masters of guerilla warfare and have trapped the great kingdom of Rigus in a decades long Hopeless War, until Rigus tries "Operation Ingress'' a mission where an elite strike team would deposit a sorcerer with a secret weapon near enemy territory. The secret weapon was a crystal prism that opened a gate to the Void a horrid location of erotic orifices, hideous mouths and eyes plus lots of greasy tentacles and unleashed an Eldritch Abomination army. The horrific slaughter that followed, resulted in the Dren offering an unconditional surrender the morning after.
- Starworld by Harry Harrison has a Space Battle between two fleets: one belonging to Earth and one to La Résistance. Up until then, all space combat was done with missiles (conventional and nuclear) with missiles being used for both offensive and defensive purposes. Energy weapons might work well on planets but their range is too short for space warfare. So they end up building mass drivers the length of the ship, launching iron cannonballs, as well as a smaller, turreted version firing explosive bullets. The opening volley from the main drivers cripples the unsuspecting Earth fleet, which are finished off by the bullets. The Earth fleet doesn't even have a chance to shoot back.
- In Carrie Patel's Recoletta (a Steampunk setting where a post-apocalypse North America has reverted to the technologies of the 1800s and earlier), the uprising against the White Nails had thrown the underground city of Recoletta into chaos. The rebel, Jakkeb Sato won the city, but his reign is a disaster and it appears Recoletta is ripe for conquest with its army and police forces in turmoil. A rival city on the surface has readied an invasion force, but an insider gets the plans to Sato, who goes to the unearthed Library of Congress and finds the information necessary to make a secret weapon. The invaders come by the hundreds in a steam train for food export and would easily overwhelm Recoletta in a surprise attack, instead they are welcomed with re-discovered Greek Fire. The closest thing to a casualty for the Recoletta army, are the many members who end up puking at the sight of so many men and women burning alive...
- The Dead Can Wait by Robert Ryan. Major Watson is asked (or rather coerced) by Winston Churchill into investigating a Disastrous Demonstration of the first tanks. The weapon is so secret Churchill has had Sherlock Holmes and a bunch of others imprisoned on an island for fear they might give away the secret. The killer turns out to be a Well-Intentioned Extremist trying to delay deployment of the tanks in the Somme so they'll remain secret until the Allies have built thousands of them.
- Buffy the Vampire Slayer: In the episode "Innocence", Xander comes up with a plan on how to deal with the episode's bad guy, who is nigh indestructible. He and the Scoobies go on a minor adventure to break into a military base, and return with a crate, which Xander opens to show Buffy. The lid of the crate blocks our view of what's inside until the climax of the episode. It's an anti-tank rocket launcher.
- Our Miss Brooks: In "Project X", the eponymous project is Mr. Conklin's secret weapon to get a promotion. It's a device that allows Mr. Conklin to listen in on and communicate with every room in the school.
- One of Bill Cosby's stand-up routines is "Buck Buck", describing a game by that name where groups of kids jump on each other and try not to fall down. Bill's team is good on defense, but the kids from the "rough part of town" claim to be even better, no-selling and laughing off their offense:
- Rough kid: What was that? A mosquito?Rough kid: A piece of paper! Somebody threw a piece of paper on top of me!
Kid on Bill's team: (dramatic pause) Come on out... Fat Albert!Rough kids: *after seeing Albert's leg come around the corner* We give! He ain't fallin' on us!
- Bendy and the Ink Machine: Joey Drew left one in the old animation studio - specifically one that can defeat Ink Bendy, who cannot be hurt by conventional means: a film reel labeled "The End". He didn't tell Henry about it before sending him to the studio. Instead, he left it Ink Bendy's throne room along with an audio recording on how to use it that Henry finds during the fifth and final chapter.
- Command & Conquer: Tiberian Series: Kane and the NOD love their secret weapons. Each game usually contains several, including invisible tanks, mole-machine APCs, cyborgs, and Tiberium-based WMDs. GDI usually isn't that big on the Secret Weapon front, but they develop one or two (the Ion Cannon from the first game being the most obvious).
- The Allies and Soviets from the Command & Conquer: Red Alert Series try their hand at it, especially in the first Red Alert and expansions, though of course, just as with the Tiberian branch, the "not reveal what it is to the audience" aspect tends to be spoiled by it being literally All There in the Manual (just as the Ion Cannon and Nod's nuke were listed in the manual for Tiberian Dawn, so was the Chronosphere and the Iron Curtain device). Hilariously, if one ignores the manual, the clearest example in Red Alert is the Trope Maker cited on the page image. The Soviet have developed nuclear weapons, which is treated with more pre-reveal foreshadowing in the Allied campaign than both the Iron Curtain in the same campaign and the Chronosphere in the Soviet.
- Gears of War saw the development of the Kill Sat project, the Hammer of Dawn for use against the Locust. Being a subterranean race, they were only slowed down. The Locust respond in the second game with a secret weapon of their own that somehow drops entire cities into sinkholes, eventually revealed to be a titanic Sand Worm.
- Umbrella corporation had a number of these in Resident Evil for dealing with bioweapons that get out of control, weapons such as a rail gun for Nemesis or a laser-firing Kill Sat in Africa. Unfortunately for Umbrella, their human foes often find them and use them for their own purposes.
- The Sniper Elite series focuses on Allied agent Lt. Karl Fairburne's attempts to destroy several Nazi wonderweapons, of which the Germans have gone through great lengths to keep secret, including but not limited to murdering innocent civilians and prisoners-of-war just for being near the bases where said weapons are being kept or tested.
- Sniper Elite V2: The V2 rockets themselves aren't necessarily the secret weapon here, given that the Germans have been using them to attack Allied targets in Western Europe for several months before the events of the game. The real secret weapon is in fact Tabun, a nerve agent being carried in select V2 rockets through an experimental warhead. Stopping the Soviets from using and obtaining such a weapon becomes the plot of the last act of the game.
- Sniper Elite 4: General Heinz Bohm, with help from Dr. Andreas Kessler, is spearheading the development of a revolutionary new wonderweapon: the Ms 300 Razor missile, a weapon designed to sink Allied shipping in one hit. As it turns out, however, Bohm didn't just make a revolutionary new weapon system, he even constructed an advanced delivery system, in the form of a Super Prototype flying wing jet bomber clearly based on the Horten Brothers' designs.
- Sniper Elite 5: Operation Kraken is actually several of these put together. Stealth-plated U-boats and Japanese I-400 class submarines that can evade Allied radar and sonar carry English-speaking Nazi spies and V2 rockets to be used against the East Coast of the United States, including Washington DC. The rockets will be launched off the US Coast, while the spies will be offloaded near New York and Boston, with both wreaking havoc onto a mainland United States that hasn't experienced the full effects of the European War.
- Star Fox 64: In the Sector X stage, General Pepper's pre-mission brief notes that reports came back stating that Andross was working on a secret weapon in this location, with Fox's crew heading in to check it out. As the stage progresses, however, it becomes clear what happened: said weapon, Spyborg, went rogue and completely demolished the base.
- In The Blue Shell Incident, Mario and Garfield's secret weapon is Metal Gear McQueen, a Metal Gear version of Lightning McQueen.
- The Order of the Stick has a Running Gag of the Monster in the Darkness, a creature who's always wreathed in shadow because Xykon wants to keep its identity a secret from the heroes (and the audience) until a dramatically appropriate time.
- The XRS is a top secret "black project". It is unclear how many people outside of the actual program are aware of its existence.
- As the Pandoric War closes in Look to the West, the UPSA's secret weapon is the Scientific Attack, which is so secret most of the UPSA government don't know about it. And those that do have been lied to. The President thinks it's a propaganda war, the Foreign Minister that it's a new targeting system. And thus they both sign off on a Deadly Gas attack without knowing.
- An American Tail: Played quite straight, when the mice build a "secwet weapon" to scare off the cats at the climax, in the form of an enormous puppet of the Giant Mouse of Minsk.
- G.I. Joe: A Real American Hero: One episode has the Joes and Cobra battling for control of a gadget literally called The Macguffin Device; it's so secret neither side even knows what the thing does. It turns out it brings imagined images to life.
- In Book 4 of The Legend of Korra, Kuvira demonstrates her superweapon (a cannon that shoots spiritual energy). But that is not her secret weapon. That would be the Humongous Mecha that said cannon is attached to.
- Wander over Yonder: In "The Rival", Emperor Awesome invites Lord Dominator over to tell her about his new secret weapon. Eventually he has the Fist Fighters bring it in, and in what can't be a completely unforeseen twist, the only thing under the tarp is Emperor Awesome himself.
- In Rose Petal Place, Nastina has two secret weapons she uses to try to kill Rose Petal: a fly swatter to catapult heavy rocks, and a unicycle lawn mower dubbed "the Blitzmower."
- Adolf Hitler: Along with his propaganda ministry, he would tell the people not to worry about the Allies advancing because the Nazi secret "wonder" weapons would save them.
- Nazi Germany actually had quite a few technological breakthroughs, though very few of them would see major use before the end of the war. The first jet fighter aircraft, for one thing. The London Gun (massive artillery gun capable of reaching London all the way across the English Channel) for another. The only reason they didn't pan out was because of Crippling Overspecialization, with such narrow purposes that such gadgets became infeasible to mass produce.
- When British intelligence first uncovered references to V-1 and V-2 weapons, they could only guess at their nature — and "death rays" and "engine-stopping rays" were higher on the list of suspects than cruise missiles or ballistic rockets.
- World War II allies:
- The atom bomb was a closely guarded secret weapon of the United States military during World War II.
- Great Britain developed what was, at the time, the most advanced computer ever built, for the purpose of code breaking. Called Colossus, its existence was classified for over half a century.
- Second only to the atomic bomb in secrecy level was the U.S. development of the proximity fuse. At the onset of the war, anti-aircraft artillery shells had to be set to go off at a predetermined altitude. If you guessed the altitude wrong, your shot was wasted. The proximity fuse allowed the shell to detect the presence of a nearby aircraft as it hurtled along, and detonate at the precise moment it needed to.
- The French regarded the Mitrailleuse (a primitive machine gun) as the secret weapon that would win the Franco-Prussian War for them. Emphasis on secret; extensive security restrictions meant that no-one had a chance to train with the weapon and develop proper tactics. After the apparent failure of the weapon in the face of determined Prussian assaults, the French believed that elan (fighting spirit) was more effective than machine guns. The Prussians (who had been on the receiving end of the Mitrailleuse) decided to concentrate on firepower instead, with tragic results for the French in the beginning of World War I.
- Something similar happened with the Canal Defense Light — the secrecy surrounding it meant that Allied commanders had little idea of its effectiveness and preferred to use more familiar weapons.
- In The Napoleonic Wars, Britain had two: the Shrapnel case shot, and the Congreve rocket. Austria had the Repetierwindbuechse, an air rifle that could fire multiple shots before reloading - Napoleon was deeply contemptuous of this weapon. Averted by Napoleon himself, who refused to purchase Robert Fulton's design for practical steamships, saying:
You would make a ship sail against the winds and currents by lighting a bonfire under her deck? I have no time for such nonsense.