[Marty is left-behind back in 1955, with the young Doc of then as his only hope to getting back to the future]
Marty McFly: I'm sorry, Doc. It's all my fault you're stuck back there. I never should have let Biff get to me!
Young Doc: Well, there are plenty worse places to be than the Old West. I could've ended up in the Dark Ages. They probably would have burned me at the stake as a heretic or something.