Great-grandparents?
I can accept all the time-travelling stuff, but the twist revelation that they have saved the lives of their own great-grandparents surely doesn't work! If the children had failed to save the lives of Sara and Tom then they wouldn't have existed in the first place to make the attempt!
It doesn't make any sense. Or does it?
Can anybody who is better at grasping plot than I am please clear up my confusion so I can stop trying to think about it?
Lovely film nonetheless, but anybody of my generation who was brought up on Back to the Future will be startled by this logical howler.