Back To The Future is the benchmark for time travel that makes plot-sense but doesn't make physics-sense. Marty goes back in time, accidentally prevents his parents from meeting, then needs to get them together again. When he does, he returns to the future which is better in several other ways due to the changes he made.
From a logical standpoint, this makes no sense. When he alters the past, he should either disappear instantly (in fact, in the past) or not be affected at all, it makes no sense for him to disappear *slowly*. When he jumps back to the future, how come things are MOSTLY the same, to the extent that his parents never say "how come your memories are all different", but some things are different? Where is the Marty from _that_ timeline? Did he go back in time? What timeline did he end up in?
But
none of that matters because you know everything you need to know in order for the plot to work. Time travel is difficult, so they don't have time to experiment, they only know what Doc can work out, which is often has big areas of "this is dangerous, don't do it", without needing to go into details of exactly
how it's dangerous. The rules are stated up front, and we're not expected to argue with Doc, we should just assume that's a voice-of-the-author. And the film doesn't betray us, it sticks comfortably to the rules stated, and never pokes into the edge cases and inconsistencies.
( Looper )