This article contains spoilers for Avengers: Endgame.
Over the last forty years, time travel has been used as a plot device in countless films, a fact the Avengers revisited during their time-hopping quest to undo the atrocities inflicted by Thanos in Infinity War. Unlike those prior movies, however, Avengers: Endgame travels a morally conscious path when it comes to establishing the rules of time travel.
Movies like Back to the Future, Timecop, and Hot Tub Time Machine maintain that it’s possible to pop back a few years and change the future for the better. Endgame, however, pulls its time travel rules from Terry Gilliam’s cult classic, 12 Monkeys.
Okay, real quick: for anyone who’s never watched the original 1995 film, stream it on Hulu ASAP. Tonally, it’s the exact opposite of Endgame, but it’s still good sci-fi.
In 12 Monkeys, the future finds humankind driven underground in the wake of a deadly plague. Using a rudimentary form of time travel, the powers that be send Bruce Willis back to the pre-disease past to investigate the Army of the 12 Monkeys, a shadowy group of terrorists (led by a wonderfully whacked-out Brad Pitt) who appear to have claimed responsibility for the spread of the virus.
Gilliam’s 12 Monkeys separates itself from the herd of other time travel movies by maintaining that, while time travel is possible, changing stuff for the better is not. Over the course of the film, no matter how much he tries to change the past, ol’ Bruce Willis is unable to correct his present. In the end, he simply finds himself an unwilling passenger in time’s unsympathetic march forward. It’s bleak, I know, but the basic point is: time travel can’t change the present because the past has already happened.
The time travel rules in Endgame are less rigid, but no less severe. When you travel back in time and start dicking around, you won’t change your present, you’ll create a bunch of alternate timelines. Those timelines might be hunky dory, but you haven’t fixed your own circumstances, you’ve just abandoned it for better ones. So, the fact remains the same: time travel can’t change the present, because the past has already happened.
So, when Thanos says he’s inevitable, he’s 100 percent right. He will have always happened. But the destruction he inflicts doesn’t have to be permanent.
Therein lies a moral at the center of Endgame (and 12 Monkeys) that is absent in the majority of other time travel movies. Back to the Future is about erasing the mistakes of the past; Endgame, by comparison, understand that the only true way to fix the present is to fix the present.