Some of the greatest battles in science fiction haven't involved dogfights or shoot-outs, but time-traveling smackdowns, with two different people trying to change history out from under each other. Like Marty and Biff, trying to wipe out each other's timelines in this clip from Back To The Future 2. As soon as you have more than one time machine, you can have timeline-altering sniper fights, and whoever can erase the other person's time line first wins. Start your paradox engines, and may the slipperiest time-trickster win!
Time Cop. Jean-Claude Van Damme is the only one who can safeguard history against those who would change it for their own evil ends. But a corrupt U.S. Senator (Ron Silver) is messing with the timeline in order to become president in 2004. Van Damme quickly figures out what's going on. But then Silver changes history some more, so when Van Damme returns to his present, everything has changed and Van Damme no longer has a job. It's up to Jean-Claude to go back once again and change the past a second time, getting rid of Silver in the process. Weirdly, this is one of the best movies about time travel in spite of its action-movie star.
(Versions of Van Damme's Time Cops show up a lot in SF, including the ChronoGuard in Jasper FForde's Thursday Next novels, and the temporal police from the 29th century, who show up in Star Trek: Voyager a few times. Stephen Hawking has famously theorized that some kind of temporal police must exist, to prevent the horrendous paradoxes that would otherwise happen. In Ken MacLeod's Newton's Wake, they're referred to as the "Quantum Angels.")
Primer. Abe and Aaron create a time machine, which requires you to lay inside it for as long as you want to go back for. They go back and start meddling with their own pasts, speculating on the stock market and tinkering with other things. But soon they're making more serious changes — knocking out their past selves and taking their places. They live through the same day or two over and over again, creating alternate timelines with subtle differences each time. Eventually, Abe and Aaron start trying to counter each other's interference, but keeping up with which version of Abe or Aaron you're seeing gets trickier and trickier.
Back to the Future Part 2. When "Doc" Brown carelessly leaves his Delorean time machine unguarded, that big lunkhead Biff goes back in time to 1955 and gives his younger self the means to become rich and powerful far beyond his pathetic dreams. Our hero, Marty, has to go back in time to 1955 for the second time in a row — except instead of changing Biff's future as he did in the first movie, he's just trying to undo the changes that Biff has already made.
Up the Line by Robert Silverberg. Jud Eliott III gets a job as a time courier, showing tourists the wonders of history. But some of his crazy colleagues start messing around with the timeline and wrecking history, so he has to keep going back and trying to fix the damage without attracting the attention of the Time Patrol. And then he falls in love with a time paradox named Pulcheria, his own great-great-great-great-grandmother, and it all goes to pot.
The End Of Eternity by Isaac Asimov. Harlan belongs to a time agency called Eternity, which exists outside of time itself. He and his fellow agents go around changing history to reduce human suffering. But then Harlan has a falling-out with his bosses over his girlfriend Noÿs, whom they want to erase from history. Harlan is supposed to help one of his colleagues, Cooper, go back to the 24th century and become the scientist whose discoveries later make the Eternals possible. In a fit of pique, Harlan sends Cooper back to 1932 instead, so he can't lay the groundwork for Eternity and Eternity will never exist. Finally, after the Eternals un-erase his girlfriend, he agrees to go back and rescue Cooper from the past — but then his girlfriend Noÿs reveals that Eternity's secret purpose is to edit history to make sure humans never colonize the stars. So instead Harlan helps her to change history so that humans discover atomic energy earlier, and start down the path of space exploration. As a consequence, Eternity ceases ever to have existed.
Lightning by Dean Koontz. Laura has a guardian angel who shows up to help her whenever she's in danger, but then it turns out other people are trying to undo the "angel's" work. Some evil Nazi time travelers are trying to destroy Laura. As Laura's son explains:
They can hopscotch around us.. They can pop ahead in time to see where we show up, then they pick and choose the easiest place along the time stream to ambush us. It's sorta like... if we were the cowboys and the Indians were all psychic.
It also contains the great line, "How can you win against goddamn time travelers?" How indeed?
Doctor Who. For a show all about time travel, Doctor Who doesn't have that many stories where the Doctor and another time-traveler are both changing the timeline back and forth, surprisingly. But the Doctor and his fellow Time Lord the Master get into some duels on a few occasions. The most over-the-top is in the comedy special "Doctor Who And The Curse Of The Fatal Death," where the Master and Doctor meet up in a castle. The Master goes back in time and bribes the architect to put a trapdoor right where the Doctor happens to be standing. But then it turns out the Doctor also went back in time, and bribed the architect even more — to put the trapdoor where the Master is standing instead.
Dirk Gently's Holistic Detective Agency by Douglas Adams contains a lot of cris-crossing back and forth in Reg Chronotis' time machine (much of which is lifted somewhat from the episodes Adams wrote for Doctor Who. In particular, the ghost of the last surviving Salaxian possesses a disgruntled literary magazine editor, inspiring him to go back in time to repair the Salaxian spaceship before it can explode, back at the dawn of life on Earth — which will have the effect of making sure life never develops on this planet. The instructions for fixing the ship are buried in the second half of Coleridge's poem "Kubla Khan." But Chronotis and Dirk Gently, our detective hero, go back to Coleridge's time and ensure he never finishes that poem, so the instructions are lost and the alien plot is foiled.
Terminator. The Terminator movies and TV show are all about people and cyborgs traveling back in time to change, or safeguard history. The machines want to kill Sarah Connor before she can ever give birth to future resistance leader John Connor, so John sends Kyle Reese back in time to protect him — and Kyle becomes John's daddy. And then, the machines send more cyborgs back to kill John, and eventually Kyle's brother Derek ends up back in our time hanging out with his friend/nephew as well. And Sarah Connor either dies of cancer or travels forward in time past her own death date and somehow avoids it. Maybe in the second season of Sarah Connor Chronicles the machines will figure out they just have to wipe out the Reese brothers as kids, and all their problems go away.
Time After Time. H.G. Wells and Jack The Ripper battle each other in the bizarre future of 1979. Once they both reach the future, time travel doesn't play that much of a part in the story — except that at one point, Wells travels forward in time three days with his girlfriend Amy, only to find Amy's obituary in a newspaper. They have to travel back again and prevent Jack the Ripper from making Amy his fifth victim. (In the end, it turns out the obituary was mistaken, and it was Amy's friend who was murdered.) And then Amy goes back to the 19th century and marries Wells, changing history at least somewhat.
Meet The Robinsons. An animated Disney film, very loosely based on the book A Day With Wilbur Robinson. Tom Selleck invents a time machine. (We'll just pause to let you absorb that piece of info.) And then a villain named Bowler Hat Guy travels back in time to sabotage a memory-scanning machine that a kid named Lewis has invented, which gave rise to all the amazing inventions in Tom Selleck's utopian future. ("Tom Selleck's Utopian Future" will be my next band name.) So Tom Selleck's son Wilbur has to travel back in time to our time, to make sure Lewis repairs the memory-scanning machine.
Crime Traveler. In this British TV series, a physicist named Holly Turner invents a time machine, and a lazy detective named Slade uses it to travel back in time and solve crimes before they happen. But in the final episode, a criminal gets his own time machine, and travels back in time to give himself an airtight alibi for a couple of murders. Slade has to travel back as well, to catch the other time traveler in the act.
Research by Nivair Gabriel