Most people have seen a variety of time-travel films, most of which cover the same type of plot: some form of accident occurs, the main character finds a way to travel back in time to prevent it. Bing bang bosh.
This film does have elements of this classic formula but it also introduces a whole new factor: trying to influence the past from the present. I won't say too much as I don't want to add any more spoilers to the ones that have already been posted but I will say that it has something more than most films of its kind.
This is definitely one to turn your brain off for because if you try to get your head round it, especially if you have a scientific background like me, you will go mad! When it comes down to it, the feasibility of the concepts introduced in the story aren't important because it's science fiction!! FICTION being the operative word.
I only didn't give this film 5 stars because in my opinion no film is perfect enough to deserve 5 stars but this one is close. Just sit back and enjoy one of the best time travel films since Back to the Future!