9. Not Knowing Who You Are - Dark City
1998's Dark City has one of the best twist endings in all of science-fiction, but that glosses over something horrible that is revealed through the course of the film. The movie begins when a man wakes up in a hotel room, but he doesn't know who he is or how he got there.
In the room is the body of a sex worker, and he must learn who he is, and whether or not he was responsible for the woman's death. As he works his way back to try and discover more about himself, he is chased by strange pale men.
These men turn out to be the ones behind everything going on in the city, which stops at midnight every night and is reshaped by the strange beings, who turn out to be aliens inhabiting the bodies of the dead. They are looking for something, and when they find it in the man, it spells their destruction.
The twist comes when it's revealed that the city is actually a spaceship, and the inhabitants were abducted at some time in the past. Their memories had been erased and rewritten night after night, meaning none of them knew who they were, where they came from, or how to get home.