Batman: Do you realize what you're asking?
Superman: I do. I want the means to stop me in the hands of a man I can trust with my life.
In an optimistic setting, this device is from a character who is aware of their Kryptonite Factor and puts the means to stop them in the hands of another (usually, in case they're brainwashed or go evil somehow). This usually implies a bond of trust since the receiver often has mixed feelings about the idea and the giver has to insist. This is also a show of good faith to a skeptic so he will allow the hero to operate in a legal grey area.
This trope is named after the ring Superman gave Batman in case he ever got out of control.
Compare Betrayal Insurance where the "stopping" character came up with it on their own; exactly how extensive this planning comes off can seriously affect the relationship depending on how betrayed the other feels. May overlap with Mercy Kill Arrangement if the results are likely to be fatal.
- Professor Xavier had the "Xavier Protocols", a set of plans on how to stop the X-Men. Xavier himself is the subject of the first entry, which includes blueprints for a telepathy-blocking battlesuit. However, these have rarely been mentioned since they were first introduced.
- In Wolverine: Origins Wolverine gives Cyclops his magical sword, which is capable of slicing through his adamantium skeleton and negating his healing abilities, in case he gets captured and brainwashed. Of course his son gains it later. Though at least Wolverine was able to use it to kill Sabertooth first (not that that stuck even a whole year).
- In Frank Miller's dark, near-future miniseries Batman: The Dark Knight Returns, Batman
actually uses his Kryptonite Ring (the cynical version)has Green Arrow shoot Superman with a Kryptonite arrow to beat the crap out of Superman. This inspired the writers to have the canon Superman give Batman a Kryptonite Ring (the idealistic version).
- The quote is a slightly truncated version from the storyline Batman got the ring, "Dark Knight Over Metropolis". In it, Superman and Batman work together to try to figure out why a homeless man was found dead with Lex Luthor's old Kryptonite Ring. Batman ultimately pieces together that the ring was actually in the possession of a woman who became manically determined to prove Clark Kent and Superman were one and the same and was killed by a bunch of random muggers, which lead the homeless man to take the ring. Batman initially kept the ring, but gave it to Superman at the end, only for the Man of Steel to return to Batman and give him the ring, feeling that he could trust Batman with such a dangerous weapon.
- This specific dynamic, Batman holding Kryptonite, appears again and again, especially in the DC Animated Universe. During the height of the Cadmus story in Justice League, Batman asks Superman what could stop the latter from taking over the world, if he so desired. Supes quips, "There's always that Kryptonite you carry around." Which makes Batman snap at him for being so flippant about so serious a topic.
- Subverted in Pre-Crisis story World's Finest #176 in which Batman appears to have Kryptonite gloves, but in reality they are fake. Curiously, before he reveals that they are props, no one -not even Supergirl- questions why he owns something that can kill his best buddy.
- There's also the piece he uses against A.M.A.Z.O. when it copies Superman's powers (and weaknesses).
Hawkgirl: Do you always keep that in your belt?
Batman: Call it... insurance. (grapples away)
Hawkgirl: And they call me scary.
- The Batman has an incredibly cynical version. When Batman got a piece of kryptonite off of Metallo, not only did he keep it, he lied about giving it back (he gave Supes a fake one and kept the real one). He did it to "get even" with Superman for finding out his identity. However, by the end of the episode, it's the idealistic version, as they have settled their differences, and when Batman offers to give Supes the real kryptonite, Superman says to keep it. It still goes to show you that Batman really doesn't like anyone being nearly as Crazy-Prepared as him.
- This is also inverted by the end of The Batman: Batman gives each member weapons that simulate each Justice League member's weaknesses because aliens have invaded and copied their powers. They call him out on being so paranoid until Superman steps forward and cites the time he was mind-controlled in a previous episode.
- A story combines the idealistic and cynical versions: after a long-term mission to destroy all the kryptonite on Earth, Superman saves one piece, which he gives to Batman for emergencies. Batman takes it to a lead-lined room in the deepest area of the Batcave... which is already filled with multiple kinds of kryptonite!
- In The Supergirl from Krypton (2004) Batman uses a chunk of Green-K to knock Kara out and bring her to the Batcave. Later, Superman of all people uses a kryptonite ring to knock a brainwashed Kara out.
- In Elseworld's Finest: Supergirl & Batgirl, Lex Luthor has a Kryptonite ring that he plans to use against Supergirl... until Batgirl steals it from him and takes it to a lead-lined vault, saying "I see it as insurance... which I hope I never need to use."
- The trope is referenced, as always in combination with Crazy-Prepared, when in the first episode of Batman: The Brave and the Bold, Jaime Reyes asks his friend who he thinks would win: Batman (no kryptonite) or mind-controlled Superman. He then informs his friend, who answered "Superman", that it was a trick question: "Batman always has kryptonite".
- The New 52 version is idealistic and reciprocal, as Batman includes himself in the countermeasures list, trusting Superman to stop him if necessary.
- In War World, the Martians prepared a cluster of miniature missiles armed with Kryptonite in case that Superman tried to seize the device that controls the eponymous super-weapon and they were tasked with guarding.
- To a lesser extent, the Shazam!! Captain Marvel is employed occasionally as the one superhero tough enough to take on a controlled Superman if necessary. Or rather, he's tough enough to stall Supes in a fistfight long enough to call his magical lightning enough times to knock out his Kryptonian foenote . Although, at least in one occasion, it was Captain Marvel the brainwashed one, not Superman.
- In the Legion of Super Heroes cartoon, Drax steals Brainiac 5's kryptonite and tries to use it on Supes, but Superman kicks him through a conveniently placed Phantom Zone portal, before he can even open the lead container.
- Pre-Crisis, Superman's heat vision could melt kryptonite, rendering it useless somehow. This usually worked only on small pieces, though; in one instance a large meteor of the stuff fell next to him and incapacitated him before he could melt it.
- Referenced in Kingdom Come, but it's noted that Superman has grown more and more powerful as he's aged and absorbed more solar energy, and Kryptonite doesn't really work on him anymore.
- Infinite Crisis subverts this when Batman tries to use his Kryptonite ring on Superman of Earth-Two, which has no effect on him because it isn't from his universe's Krypton.
Kal-L: "But the Kryptonite here isn't from my Krypton. It doesn't hurt me... physically, at least. But that ring [...] represents the paranoia and mistrust that will destroy your world if you let it."
- The Third Kryptonian reveals Batman kept secretly working on Superman-stopping plans after Infinite Crisis... which is because he isn't amused when Superman mentions he is already aware of it.
- In All-Star Superman, the Man of Steel helped Leo Quintum make three, including a Kryptonite weapon, a Phantom Zone Kill Sat, and a super serum that can temporarily turn a person into Doomsday.
- In the Crucible story arc, the titular super-hero academy develops fail-safes for every new student, tied to their unique traits, just in case they turn rogue. In Supergirl's case, it is a device that blocks her solar energy's reserves. They are supposed to be a security protocol in case that some student attempts to destroy the Academy, but the Big Bad uses them to force rebel students to do his bidding.
Korstus: "We develop a fail-safe for every new student, tied to their unique genetics. A fail-safe that ensures their... cooperation... should the need arise. In your case, a device that inhibits the ability of the solar energy stored within your cells to be delivered to the rest of your body. Much the way you might starve a plant by keeping it in shadow."
- The Hunt for Reactron, the Science Police K-Squads are equipped with powered armors, fire plasma bursts, sonic weapons, neuro-disruptor energy nets... to take Kryptonians down.
- In Way of the World, villain Dolok has a Kryptonite collar in case that he needs to subdue a Kryptonian.
- Injustice: Gods Among Us: In Superman's ending, it's revealed that the Regime-Superman's ease at conquering the world only made him even more terrified of what could happen if he ever lost control of himself. He makes sure this doesn't happen by having a Kryptonite bomb implanted by his heart, which will instantly kill him should this happen. The other members of the Justice League all take turns carrying the detonator for it. Except for Batman, who was denied a shift.
- In the Back Story of Ex Machina, The Great Machine (now the mayor of New York City) gave his two Secret Keepers devices to cancel out his ability to talk to machines. As was his habit, he explicitly referenced the Superman mythos as he did so. As we later learn it's just junk made out of old garage door openers and doesn't affect his powers at all. He lied and gave them to his Secret Keepers so that, if they ever betrayed him or were forced to turn against him, their plotting would depend on something that doesn't work. (Though a major plot hole is that it did work in the first story arc.).
- There's no actual power-cancelling device, but Avatar: The Last Airbender - The Promise begins with Zuko asking Aang to kill him if he ever starts following in his father's footsteps. This comes directly after the series finale, in which Aang found a way around executing Zuko's dad (who is pretty much the most despicable man alive). He's not exactly thrilled, but Zuko insists he promise. Cut to a year later when Realpolitik has forced Zuko to enforce a slew of distinctive Ozai-like decisions. Cue drama.
- Here Comes The New Boss: In an attempt to gain Panacea's trust after she figures out her identity as Butcher XV, Taylor meets up with her later and hands her a journal full of all of the Butcher's powers and histories, to be given to the Protectorate in the event that she loses control or she dies and a new villain inherits the power of the Butcher.
- In History Teaches Us, Thorin has to open up to Thranduril and Legolas about the gold-sickness that runs in his family because he wants to assure them in advance that anything he might say to them in future that goes against his current deals with them will be because he isn't well rather than because he's currently lying to them.
- In The 100 fic "Twisted Steel", Clarke and Anya first bond as allies rather than enemies when Clarke has to ask for Anya's help in removing her artificial arms, as the limbs suffered water damage during their escape from Mount Weather. Anya explicitly tells Clarke that there's no shame in seeking aid from another, but also thanks her for the trust she's shown Anya in revealing such a weakness as she demonstrates how to remove the limbs from her body.
- In Buffy the Vampire Slayer/Supergirl crossover The Vampire of Steel, Supergirl lends her friends a chunk of Kryptonite in the event that things go south during the Final Battle.
- In The Dark Knight, Batman gives Lucius Fox sole control over the cell phone sonar system and the ability to destroy it so that Fox will trust him to use it only against the Joker. Though Fox didn't actually find out about the second part (cryptic instructions on how to "turn it off") until the Joker was caught.
- In Avengers: Age of Ultron, Iron Man and Bruce Banner discuss how they developed "Veronica", the Hulkbuster armor, as a "last resort" to stop the Hulk. So when it needs to be employed, it serves this function perfectly.
- In Corpies, Titan reveals that the government has a Super on standby, who has the ability to cause deadly brain aneurysms, in case he (Titan) ever goes rogue. Given Titan's Adaptive Ability, this is currently the only known way of reliably killing him (and even this will fail if he's ever attacked by a similar but less lethal ability, as his body will adapt to all similar attacks). When the others incredulously wonder why the government would tell him about this, he explains that he was the one who suggested it to them.
- In the Inheritance Cycle, sharing your true name with another is a BIG deal, as it allows them to use spells on you and mind control you with impunity. Eragon doesn't hesitate to share his with Arya at the end of Inheritance, who reluctantly returns the gesture.
- In Star Trek: The Next Generation, there are various methods created to deal with Data to control him in case of possession or other matters. There is an off switch located on his spine that only a few know about, and some Applied Phlebotinum to disable him for extended periods.
- In Terminator: The Sarah Connor Chronicles, Cameron installs a bomb in her head and gives John the detonator, in case she ever becomes a threat to him. She has good reason to worry about this because it's happened to her before and nobody (not even the Terminators themselves) knows what causes a reprogrammed Terminator to suddenly revert to its default programming.
- In Smallville:
- Oliver Queen kills Lex Luthor and takes his kryptonite ring, confirming his role as Smallville's Batman. (Well, except for the Thou Shalt Not Kill thing.)
- The Kents are also known to keep pieces of green kryptonite around. It was first used to counteract red kryptonite, but later his parents and Chloe Sullivan uses it a lot to stop him whenever he is mind-controlled (Chloe saves Lex from Clark in Hypnotic), possessed (Jonathan saves Chloe from Dawn possessing Clark in Spirit) or is otherwise not in his right mind (Chloe saves Jonathan from Clark when he is made paranoid by silver kryptonite in Splinter).
- Double Subverted in season nine. Clark gets mad at Chloe when he discovers she's been stockpiling kryptonite weapons. Then it turns out she was doing it in case he had to fight Zod and the Kandorians.
- Kamen Rider Build: Sento created an anti-hazard switch that he handed to Misora. She was supposed to use it in case he lost control of Hazard form and there wasn't anyone to stop him. He told her that it will just knock him out of transformation. She correctly deduces that this is not all and manages to make tell her the whole thing. It will also destroy the Build Driver and kill him. She is very, very mad at him for making her promise she will murder him if the need arises.
- The Flash (2014)
- Cisco creates the Cold Gun in case Barry turns out to be dangerous, like the other metahumans. Unfortunately, it gets stolen and later gets into the hands of Leonard Snart, a.k.a. Captain Cold. Both Wells and Barry tear Cisco a new one for making the gun.
- The reason why Season 5 Arc Villain Cicada became such The Dreaded among metahumans: he has a special dagger that saps all dark matter energy out of any metas in the vicinity, depowering them and becoming vulnerable to Cicada's attacks. Fortunately for the heroes, there's one meta on their side who's revealed not to have her powers derived from dark matter, so she is immune to the dagger's effects: Killer Frost. This is also why Hidden Agenda Villain Eobard Thawne is actively helping the heroes fight Cicada (through his unknowing apprentice Nora, the Flash's daughter). The dagger is strapped to him in the future, leaving him powerless while he awaits death row.
- Dark Souls III: In the game's backstory, Yhorm the Giant was descended from a long line of brutal conquerors. But when he became king of the Profaned Capitol, he desired to be a good king to his people. To engender trust in him, he provided the people with an enchanted sword to which he has a fatal weakness, so they could kill him if they ever thought he was doing a poor job. All indications are that the sword was never used against him. He gave a second blade to his trusted friend Siegward of Catarina, making Siegward promise to strike him down should he ever fail his duties. If you complete Siegward's questline, he will wield this blade in battle alongside you, because Yhorm has abandoned his duty as a Lord of Cinder.
- It is eventually revealed in Final Fantasy VIII that Edea had feared being taken over by an evil sorceress, and with her husband created a fighting force to oppose her in that event, which has already come to pass by the game's beginning.
- Downplayed in El Goonish Shive. When Sarah volunteers to be Tedd's lab assistant, Tedd feels that if they are to work together, he needs to make up for the catgirl incident. So, as a show of trust, he gives her a watch that can let her shapeshift into a clone of Tedd. While he doesn't particularly mind her taking a peek, he points out that there are still all sorts of ways Sarah could use Tedd's form to embarrass or humiliate him if he ever steps over the line again.
- In a flashback arc of Everyday Heroes, Jane, recently out of prison, laments that people aren't inclined to trust an ex-con. Mr. Mighty says that he trusts her, and she points out that it's easy for him to trust someone who can't hurt him. A bit later, he gives her one of his superpower-dampening pills to prove that he trusts her when she can hurt him, then asks her to marry him. She accepts his proposal after flipping the pill into his mouth to enable them to have safe sex.
- Whateley Universe: As of "Ayla and the Mad Scientist" we now know Phase has at least several plans to take down every one of his teammates. And a bunch to take down himself. And his teammates know his 'official' weakness in the school records is a fake. He wrote up a list of real ways to stop himself and gave it to Lancer.
- SilverHawks: Each of the SilverHawks has an emergency off-switch to be used if any of them went berserk for unforeseen reasons.
- Some people have said that the second amendment of the American constitution, the right to bear arms, is there partly in case of the government being taken over by an oppressive tyrant of some kind. In this event the people would be able to overthrow the regime because they would all be equipped with guns. This can be seen as the founding fathers giving future generations of citizens a kryptonite ring. There is substantial disagreement over whether this was a true intention as well as just what the actual effectiveness of it is in the 21st Century given the now extremely broad gap between civilian and military weaponry, but the thought of the American government massacring its own citizens would certainly not paint the picture of a perfect democracy, and most American politicians (even the most vocal proponents of gun control) do not want to enact a complete gun ban on the premise that doing such would incite a form of armed rebellion. Words don't stop bullets very well.