time travel & paradox

Marty McFly reverses into 1955 in Back to the Future and has to get his own parents together before he stops existing. About Time tells a young man the men in his family can go back, and lets him learn what that fixes and what it does not. Terminator 2: Judgment Day sends a machine back to protect the boy the first one came to kill. The rules are strict, and breaking them costs somebody.

The requirement is that causality is actually at stake. A film set in the past is a period film; a film that remembers is Memory Loss or Nostalgia & Longing. This term wants the second pass — the character who has been here before, and whose knowledge of what happens is itself now part of what happens.