As I’ve mentioned before, time-travel films have logical paradoxes aplenty. Most typically the grandfather paradox where changing the past eliminates the time-travel trip ever taking place.
The standard solution to this is that the time traveler didn’t really change history, she just created a parallel world. In the old world, everything remains as it was; the happy ending is for the new timeline.
This doesn’t always work. Some movies, such as The Black Knight, have other logical problems. The horror film The Caller (2011) which I caught last weekend has someone changing the protagonist’s past and affecting the protagonist’s present—so it’s definitely history being changed, not world-line jumping.
A bigger problem, as I noted at the link, is that it doesn’t work dramatically. Take Against Time, in which a drunken, broken-down Robert Loggia travels back in time to stop his teenage self from causing a tragedy. After he succeeds, his personal history changes so he never makes the trip; instead he makes an unrelated trip at the end of the film, showing his new alt.self is happy, healthy and contented.
If all he did was create a parallel world, then somewhere out there in the old timeline he’s still drunken, broken-down and miserable. In that timeline, his younger self will still make a stupid mistake that gets two dozen kids, including his own son, killed. Nothing’s changed.
Dramatically speaking that just doesn’t work. It might work in fiction, but it definitely doesn’t when I’m actually seeing the characters up on the screen, flesh-and-blood rather than printed words. I want Loggia to heal himself. I want him to avert the tragedy. If it’s just a new timeline it’s a cheat.
Likewise, if all the cast of Terminator 2: Judgment Day did was create a new timeline without the machines (of course T3 subsequently made it clear they hadn’t beaten Skynet anyway), that’s a good thing (millions of people live without going to Skynet’s death camps) but it’s still much less satisfying. It’s the problem Larry Niven raised in his short story “All the Myriad Ways”—if every action creates multiple timelines then you accomplish nothing—it’s just random luck which timeline you wound up in (as Crisis on Two Earths points out).
It’s similar to my reaction to Family Man when I realized Cage’s kids were erased from existence when he left the magically created alternate world. I wouldn’t have batted an eye if it happened in a book, but on-screen they’re so real ….
So unless a film specifically invokes parallel worlds, I’m going to assume there aren’t any. Paradox or not, history changed. Screw logic, movies work better that way.