Temporal Paradox: Now it gets complicated…
1. Characters go to the past! In the past, they change history: If they do so by accident, it well may end the story with a Karmic Twist Ending; alternately, it will set the real plot in motion by requiring the characters to Set Right What Once Went Wrong.
2. On the other hand, they may have set out to change history intentionally, so that the events that create their future/present — and, thus, the conditions that prompted them to go back in time — never happened, basically the same set up as above, but without the initial “accident.”
3. Characters go to the future! Upon returning to the past, they are able to fight fate and prevent the events of the future (seeing which prompted them to try to prevent the events of the future in the first place) from occurring.