The Original Spaghetti Western (unless you count Yojimbo, the Kurosawa movie it was lifted from). Clint Eastwood, on a break from the successful TV show Rawhide, became The Man With No Name (whose name is Joe in this film, but whatever).
A stranger comes in to town riding a mule. Forced to defend the mules honor, the stranger kills four of the gunfighters working for one of the two criminal families who run the town. Soon the stranger is working for one of the families then the other. Eventually, the whole town is at war and the stranger is back to settle a score.
We don't want to give away the ending, but if you've seen Back to the Future 3 you kinda know the twist.
And did I mention the classic Ennio Morricone soundtrack? Refresh yourself
An iconic motion picture that became the first of an unintended trilogy and a cultural touchstone across generations.