New T4 Trailer

[MetalMilitia]

You can timetravel back in your own time line but once some variable changes it must split to a new one. From then on you can only travel back along this new time line.
Because this new time line didn't exist when the first time traveller came back it didn't affect it but in subsequent time travels the traveller would have to follow the new time line.
Yes this makes sense. That's how it works.

Back to the Future physics are slightly different and far beyond the scope of my diagram.