Time Travel Scenarios
1)One can only travel along a fixed timeline.
The idea is: the time traveller is a tourist that has no agency over the events unravelling.
First variant: he/she can only observe what is going on. (cf. The tourist disasters of 'Fringe')
Second variant: no matter what the time-traveller does, events have been pre-ordained and will occur regardless. ('la Jetée'->'The 12 Monkeys'; 'Donny Darko'; 'Terminator 3')
2)Actions change the future.
In this scenario, the time traveller can act and therefore alter the chain of events. (cf. Bradbury's 'A Sound Of Thunder'; 'Primer'; 'The Butterfly Effect'; 'Happy Deathday'; 'TimeCop')
The German series 'Dark' plays the second hypothesis against the first one.
...And then throws in the third logic for good measure!
3)Creating alternative chains of events.
Every action creates a new chronology that branches out from the original timeline: it's the multiverse theory. (This is what the Terminator makers resorted to in the 5th? Movie, having pretty much exhausted the second train of thought.).