Terminator Twosome

Two warriors are sent back in time. One to change history, one to ensure it takes its established course. Or, at least, to prevent the first one from changing things. A game of cat and mouse ensues.

Can also be thought of as Evil Counterpart IN TIME! If it does not overlap with Set Right What Once Went Wrong, it will instead overlap with Make Wrong What Once Went Right: In this case, the historical event in question goes as it originally did in the "good" timeline, and is averted in the "bad" timeline.

Named, of course, for the Terminator movies, wherein a robotic superassassin that looks like a normal human being is sent back in time to pre-emptively kill the future leader of the human resistance in a Robot War. Paradox schmaradox! Compare Scry vs. Scry, where it's oracles doing this with clairvoyance.

Anime

 * Mahou Sensei Negima has a complex version of this: A Well-Intentioned Extremist from the future goes back to Set Right What Once Went Wrong, bringing her to the storyline's present. Unfortunately, her goals would cause major trouble for the main characters (turning into an ermine can put a crimp in anyone's plans), and she sends them a week forward in time to a future in which she's already won. Good thing Negi can go back in time too...
 * Dragon Ball Z: Cell and Trunks, although they come from different timelines. Trunks had no idea who Cell was, and Cell killed the Trunks of his timeline before going to the past.

Comic Books

 * In X-Men, Cable came back in time to prevent the future he grew up in. Stryfe, his evil clone, then came back in time to stop him -- not so much because he was hugely enamoured of their home era, but rather because he lived to screw with Cable's life. Oh, and then Cable's evil adopted son came back as well...
 * Slightly less directly, Bishop originally came back in time hunting a group of time-traveling criminals, but then decided to use his new location to protect the X-Men and prevent his home timeline. The criminals he was hunting don't seem overly concerned with this.
 * He only joined the X-Men after he had killed off almost all the criminals he came back to find. The leader of the survivors was a recurring villain for several years.
 * And then there was the Legionquest story, which culminated in Professor X's evil son (yes, this is something of a recurring theme for the X-Men) traveling back to the 1950s in order to kill Magneto before he becomes powerful. An entire team of X-Men go after him. Between them, they screw things up even worse.
 * Current X-Men storylines have Cable jumping into the future to protect a girl he believes will become The Messiah, while Bishop wants to kill her because in his original timeline she becomes The Antichrist.
 * Also worth noting is the fact that, hilariously, Cable seems to react to every single piece of legislation worth mentioning in any comic with the assumption that it's the first step on the way to his horrible future. Civil War alone contained at least three separate instances of this.
 * To be fair, he's from the year 4000. He clearly has no idea what he's talking about...ever.
 * Iron Man and Doctor Doom for some reason do this often. They wound up in Camelot once, and another time they inverted the trope by traveling to the future (after Merlin has awaken and King Arthur has reincarnated). Often, though they end up taking a third option and working together to get back.
 * And again in Bendis' Mighty Avengers. It helps that both Doom and Stark have nothing to gain from changing anything in the specific past periods they enter, and both would rather keep the present they have. Seeing as it's dangerous to fuck with reality and all that.

Film

 * Terminator, obviously.
 * In the first movie, a robot is sent back to kill Sarah Connor before she even gives birth to John Connor, and a Badass Normal human is sent back to protect her.
 * In the second and third movies, a shapeshifting robot is sent back to kill John Connor, either as a child, or shortly before the onset of the Robot War. Each time, a reprogrammed robot like the evil one in the first movie is sent back to protect him.
 * In the third film, because John Connor cannot be located, the T-X is instead sent after who will become John Connor's top lieutenants and wife. Finding John Connor was just a bonus.
 * Ignore the Fridge Logic about not being able to send a second terminator into the events of the first movie!
 * The comic crossover with Robocop written by Frank Miller had a human sent back to kill the former Alex Murphy before he could grant Skynet sentience, and a Terminator sent back to protect Robocop by any means necessary -- whether he wanted it or not.
 * In The Sarah Connor Chronicles, another evil robot is sent back to kill John Connor as a teenager, and a reprogrammed robot who looks like a cute waif of a girl is sent back to protect him. Over the course of the series, other robots and human resistance fighters are also sent back.
 * They even have conflicts between the many terminators sent back, if their tasks conflict with one another. For example, a terminator is sent to kill and replace Special Agent James Ellison. The attempt is interrupted when Cromartie, a terminator sent to find and kill John Connor, saves Ellison and destroys the other robot. When asked why by Ellison, Cromartie simply says that Ellison will lead it to the Connors. Not under duress but because Ellison is looking for them himself.
 * Demolition Man inverts the trope, sending two people from the present into the future. One is a criminal "mistakenly" unfrozen from Cryogenic Sleep, the other is a present-day cop also in cryogenic sleep for a crime he was framed for, unfrozen to deal with someone simply too evil and dangerous for the wimpified future cops to handle.
 * In one of the Austin Powers movies, when Doctor Evil returns, Austin Powers himself is unfrozen to deal with him.
 * The first sequel plays it straight, with Austin following Evil into the past.
 * Likewise, Time After Time has HG Wells traveling into the future in pursuit of Jack the Ripper.
 * Time Cop features Jean Claude Van Damme as, well, a Timecop who travels to the past to apprehend criminals who threaten to change the timeline.
 * Trancers: When an evil psychic goes back in time to present day Los Angeles, Jack Deth is sent back to stop him.
 * Captain America 1990 with Captain America and the Red Skull being revived in the future.
 * Happens in Back to The Future Part II, when Marty and Doc travel back to 1955 to undo Old Biff's tampering with the timeline.
 * At the end of Two Thousand and Nine Lost Memories, Saigo and Sakamoto end up in the past struggling to avoid or ensure the assassination of Ito Hirobumi.

Literature

 * In The Apocalypse Troll by David Weber (of Honor Harrington fame), an alien-created cyborg-modified human is sent back in time to destroy humanity before they develop FTL space travel. A human space-naval officer also goes back in time to stop it. Also somewhat of a subversion, as it started as two entire Standard Sci-Fi Fleet battle groups fighting both before and after the actual time travel, but each side only had a single survivor left after all the nukes finished flying so this trope still applies.
 * In Drakon by S.M. Stirling, a genetically engineered superhuman woman accidentally travels through a wormhole from an alternate future back to a world resembling our own present time, and promptly begins trying to find a way to re-open the wormhole and bring through a conquering army of more genetic supermen. A lone Hollywood Cyborg human is also sent back through a similar wormhole to stop her.
 * In the Belisarius Series by Eric Flint and David Drake, two artificial intelligences from opposite sides of a far-future war get sent back to the early middle ages, one to try and alter the timeline so the evil side wins the future war, the other to try and stop the first one, via recruiting local Badass Normal allies. A semi-subversion, since both sides are attempting to change the timeline. Indeed, both rely on massively altering the cultural and technological evolution of the world; they just each want history to evolve in a different direction. This is because, after the changes already made by the "bad" AI, the timeline has already been irrevocably changed, so the "good" AI needs to insure that the right culture comes out on top.
 * Harlan Ellison's The Outer Limits episode "Demon with a Glass Hand" may have created this trope. In fact, the producers of the first Terminator movie had to pay a settlement to him following a lawsuit.
 * Well, sort of. While "Demon with a Glass Hand" is recognized as one of the inspirations for Terminator the settlement was actually over claims that Cameron had ripped off another Ellison Outer Limits episode, "Soldier", which also fits this trope.
 * The Discworld novel Night Watch features Sam Vimes and a criminal both accidentally going back in time, and Sam Vimes has to stop him from changing history for the worse - while wondering whether he can morally allow history to run its course when it means innocent people dying that could be saved if he used his knowledge from the future to make a few changes of his own.
 * Used with a ridiculously complex plot in Animorphs Megamorphs 3: Elfangor's Secret. In brief, a human (John Berryman) who is mind-controlled by an Alien Invader (Visser 4) gets a time machine, and the heroes (4 human teenagers, a human-brained hawk, and an alien, each of whom can change into animals) follow him through time. As part of some elaborate treaty involving god-like beings, the heroes become immortal after  Because of Visser 4's influence on time, the course of history is changed, to the point that in World War II,  After much debate about the ethics of changing the course of time once more, the heroes decide to
 * The Star Trek the Next Generation Expanded Universe novel Imzadi has an aging Riker trying to change the past, with Data trying to keep it as it was . When the characters realize this at the end and ask the Guardian of Forever why it didn't say so sooner (potentially saving them all a lot of trouble and/or Angst), it replies (literally) "You Didn't Ask."
 * In the Strange New Worlds story "God, Fate or Fractals"'' it's Wesley Crusher versus a pair of Temporal Investigations agents.
 * Dies Irae - Heljanita wants to change the past, and Darkscar wants to unchange it.
 * Rebel in Time by Harry Harrison. A racist colonel steals the design of the Sten submachine gun and travels back in time to change the course of The American Civil War. He is pursued by another officer, whose task isn't made easier by the fact that he's black.
 * In Tim Powers' The Anubis Gates, a millionaire discovers the existence of time portals and uses them to visit the past. A scholar brought along for his historical know-how discovers the millionaire's nasty ulterior motives, and must stop both him and the villains who'd opened the time portals in the first place. Subverted twice, in that 1) the millionaire isn't looking to change documented history, but to become a power behind the throne; and 2) the historian isn't sent back to preserve anything, and initially is only looking for a way to get back home.
 * For King And Country, by Robert Asprin and Linda Evans, features  a Terminator Twosome of an IRA agent traveling back to Arthurian times to change history in Ireland's favor or simply punish England, and a British soldier trying to stop it. They go all the way back to around 500 AD or so and share the bodies of people close to King Arthur. It seems like a Stable Time Loop and/or Tricked-Out Time, but the ending is a little ambiguous. Meanwhile in The Future, their bodies remain in a comatose state while they are in the past.
 * An unintentional example in a Russian novel, where two Russian cosmonauts somehow end up in the past during the decline of the Roman Empire. One of them gets captured by barbarians, while the other one ends up becoming a Roman legionnaire. Eventually, the latter becomes the primus pilus (senior centurion) of a Roman Legion and is determined not to let the Empire fall, while the other manages to become the chieftain of the Germanic barbarians who captured him. You can see where this is heading.

Live Action TV

 * The Sarah Connor Chronicles. See above under "Film".
 * Quantum Leap, in later seasons, features an Evil Leaper who tries to undo the positive changes in the past wrought by Sam Beckett.
 * Time Trax is a variation. The hero was sent back into the past to retrieve several escaped criminals who escaped to the past with the help of a Mad Scientist. This trope applies since the Mad Scientist himself also escaped into the past, and several times his schemes in the past threaten to change the timeline, and the hero must stop him.
 * This was the central idea of Tru Calling, though by the time it was revealed that this was the case and the conflict between Tru (change the past to save people) and her counterpart (keep this past the same and let people die) properly introduced the show was on the verge of being cancelled, so the idea was never fully developed.
 * A major plot in the first seasons of Star Trek Enterprise was the Temporal Cold War, in which various future factions used agents (some of them sent from other eras, others natives of the 22nd) to try to manipulate "historical" events in their favor.
 * The plot of the Star Trek Deep Space Nine episode Trials and Tribble-ations. The villain of the TOS episode The Trouble With Tribbles attempts to assassinate Captain Kirk in order to alter the outcome of that episode, and our heroes need to stop him.
 * Mirai Sentai Timeranger and its adaptation Power Rangers Time Force are about mutant criminals coming back from the year 3000, and the Time Police right behind to stop them.
 * Kamen Rider Den O features Imagin going into the past to wreak havoc with Kamen Rider Den-O time travelling to stop them.
 * In Charmed episode 4 season 3 "All Halliwell's Eve", the sisters are send back in time to save one of their ancestors; at the same time, team evil is sending back their main agent to prevent their birth.
 * The whole concept of the oft-mentioned Time War in the new series of Doctor Who, with the Time Lords themselves waging war against a galaxy-spanning Imperial Dalek fleet. The war was so big, most other species were too technologically underdeveloped to even notice.
 * On the other hand, this makes for an incredibly depressing Downer Ending in one of the Big Finish Doctor Who audios, Flip Flop...
 * Series two of Ashes to Ashes becomes this, with Martin Summers trying to Set Right What Once Went Wrong from his perspective, but is actually Make Wrong What Once Went Right for everyone else. Alex decides that acting as the Time Police will enable her to get home.
 * The 4400 were abducted by people from the future and returned to the present to Set Right What Once Went Wrong, while members of the anti-promicin conspiracy known as The Marked were sent back by a different future faction to thwart them.
 * The Supernatural episode "The Song Remains The Same" ultimately turns out to be a Terminator Threesome. First, Anna goes back in time with the intention of averting the apocalypse by killing John and Mary Winchester before Dean and Sam can be born. Sam and Dean, with Castiel's help, follow her in order to save their parents from Anna and from their fates in the original timeline. Finally,  goes back to ensure that history takes its established course.

Video Games

 * This is what happens in a standard two-player game in Achron.
 * Pokémon Mystery Dungeon: Explorers of Time and Darkness plays this about as straight as you can get.
 * However
 * In Timeshift, the Big Bad is a disgruntled scientist who uses a time-traveling suit to travel to the past and use his vast scientific knowledge to Take Over the World and establish a fascist dystopia. The player is a second scientist, also equipped with a time-traveling suit, who follows the first scientist through time in order to stop him.
 * Blaz Blue:
 * Inverted like crazy by Brutal Legend.
 * Played with both ways in Dark Cloud 2: Emperor Griffon, who resides years into the past, is doing battle with La Résistance 100 years into the future. In order to eliminate them, he sends out his agent into the present, to destroy his enemies' settlements and erase them from existence. Enter Monica, from 100 years in the future, whose Cosmic Keystone allows her to travel back to the present. With the help of the present-day protagonist, Max, she restores her allies' "Origin Points" and preserves future history (and, in one notable case, improves upon it.) Of course, Griffon's agent Gaspard will try to thwart their efforts....
 * The Legend of Zelda: Oracle of Ages has Veran, Nayru, Link, and Ralph, in a bizarre time-traveling quadrangle of Big Bad, MacGuffin Girl, the hero, and The Scrappy.
 * Also happens in towards the very end of, where.
 * Earthbound displays this right at the beginning of the game. Giygas chose to stop Ness while he was still weak by sending a Starman Jr. back in time to kill him directly. Luckily for Ness, a time-travelling alien named Buzz Buzz found him first to protect him. Unfortunately, Buzz Buzz dies shortly afterward. In true Terminator fashion, it is this time-travel attack that kicks off the plot and sets Ness on the path to defeat Giygas.
 * The central premise of The Journeyman Project.
 * Also the premise of the Inform game Jigsaw, although neither side is really in the wrong.
 * In Millennia: Altered Destinies, the player is given a timeship by a hooded alien to guide four races in the Echelon Galaxy in order to stop the expansion of the evil Microids. One of the major obstacles is an alternate version of the player recruited by the Microids to sabotage the player's work. Like the player, he cannot be killed.
 * Time Hollow plays a variation of this. Upon learning that the past has changed, the protagonist tries to undo the antagonist's interferences in time through time portals. The biggest conflict is.
 * Magical Girl Lyrical Nanoha As Portable: The Gears of Destiny has Well-Intentioned Extremist Kyrie Florian jumping back in time to retrieve an artifact that could help her scientist father save their dying planet. Idiot Hero Amitie Florian then jumps back in time to stop her, since as said father mentioned, interfering with the time stream could lead to even worse consequences than the destruction of their evacuatable planet..

Web Comics

 * The "Surreptitious Machinations" arc of General Protection Fault draws heavily from The Terminator, taking place in a future where Empress Trudy has conquered the world, and where Todd goes back in time to prevent the future from coming about, while the Empress herself goes back to stop him. Interestingly enough, it's revealed at the end that . A Terminator comes back to the present to deal with Todd, but fails early on and serves as a way to.
 * SSDD has this with two factions, the Anarchists who are trying to orchestrate the past to result in an economic collapse leading to their rise in power, and Dr. Cook's people who are trying to avert that. Cook's group has one cybernetically-enhanced Super Soldier (Tessa), while the Anarchists send a series of robots and clones.

Western Animation

 * A two-episode arc from the '90s X-Men cartoon involved Bishop traveling from the Bad Future to the present day, attempting to prevent the outbreak of a mutant plague. His actions backfire, however, and result in the deaths of the X-Men and the complete extinction (rather than just decimation) of mutants--so Cable comes from an even further future to stop Bishop.
 * Cause the mutant plague or stop the mutant plague? Nah, Cable takes a third option.
 * You've also got the more basic variety in most Time Travel episodes. First it's Bishop and Nimrod, then it's the Cable and Bishop thing, then it's Bishop and Fitzroy, then it's Shard and the various agents of Apocalypse (though they're from the present, Apocalypse is the version from Cable's future.)
 * Replace individual with faction and this was the plot of the third season of Beast Wars, when it turns out Megatron's plan to go back in time to destroy Optimus Prime didn't go as completely off the rails as initially thought.
 * The second series of Gadget Boy and Heather, "Gadget Boy's Adventures in history": Heroic trio versus four villains.
 * In a variant on Teen Titans, Warp from the far future is followed through a time-gate by Starfire from the present, and she winds up hunting for him Twenty Minutes Into the Future Bad Future.
 * One episode of Megas XLR revolves around Gorrath's second-in-command's plan to kill a current-day ancestor of Kiva, as Kiva and Jamie try to protect her.
 * The show's main plot has shades of this as well: Kiva and MEGAS are sent back in time (further back than expected) and the Glorft commander followed her.
 * Superman the Animated Series has a crossover with the Legion of Super-Heroes where Cosmo Boy, Saturn Girl and Chameleon Boy protect a young Clark Kent from a time-travelling Brainiac. Clark winds up teleporting Brainiac into the sun.
 * Jackie Chan Adventures did this with a Future Badass version of Jade and Shendu's Kid From the Future, Drago, in J2. Future! Jade's arrival was even accompanied by Terminator-style music.

Real Life

 * In the now-defunct Back to The Future: The Ride formerly at Universal Studios, Doc Brown sent YOU back (and forward) in time to prevent Biff from messing up the timeline.