Analyses of the dark implications of Back to the Future have been done to death. But this is the internet, and if I see a dead horse, then I’m sure as hell going to beat it. But I want to do something more. Most of the aforementioned analyses only refer to the first movie, but I’m going to look at the trilogy as a whole. If we pick up immediately after Doc Brown flies off in his time-travelling train, we find the epilogue is quite…unfortunate.
But a word about the train first. If it’s been a while since you’ve seen the movie, you probably remember that a train smashed the Delorean, then Doc Brown showed up on a flying, time-travelling train, with his wife and kids in tow. You might have forgotten that right after the Delorean is destroyed, Marty says, “Well, Doc. It’s destroyed. Just like you wanted.” The dangers of time travel and messing with the past was a constant theme in the trilogy, and Doc Brown realized the havoc his creation could cause if it ended up in the wrong hands. So when he comes back in a new time machine, the best case scenario is that he is a huge hypocrite.
But let’s put that aside. Doc Brown flies off into the sunset, and Marty goes back home. Until he is taken away be the government. You remember the terrorists from the first movie? Yeah, that storyline never really got wrapped up. In fact, their van crashes into a film development booth, which probably wouldn’t kill them. So the police get there and arrest the terrorists. In exchange for leniency (this was pre-9/11) they give up the name of the guy who got stole the nuclear material: Dr. Emmet Brown. They also mention that there was a kid with him. It wouldn’t be hard to connect Marty with Doc Brown, as even the high school principal is aware of the friendship.
So Marty is brought in for questioning. And he can’t adequately answer any questions about his own past, because he didn’t grow up in this timeline. He grew up in a world where his family was timid and unsuccessful. The events and people that populated that life occurred differently in his current reality. If he tried to answer their questions, he’d come across as delusional. Even if he told them about the time machine, they wouldn’t believe him. Except…
The only reason no one knew about the Delorean is because it was kept quiet. Between 1885, 1955, 1985, and 2015, only one person found out about it that they didn’t want to. Because a car, even a flying one, could be easy to hide. But a train? It’s impossible that no one would notice a flying train. Which, might I add, was built only with parts that existed in the late 19th century.
So Marty would become a valuable asset to a government that now has to worry about a madman hurtling through time and space with his family in a flying locomotive. Because Marty is the only person who has any experience with time travel at all (and is guilty of aiding and abetting Doc Brown), he would be held indefinitely and used to help capture his friend.