In the battle a young mutant has the ability to send someone's soul back in time to their younger body and warn the mutants of where the attack will happen, which essentially means it will never happen and the mutants continue to run for their lives.
The cat and mouse chase of humans and mutants will never end and so Professor X with his arch enemy Magneto enlist the help of this small group of mutants so that Kitty Pryde can send Professor X back in time.
In the end it is Wolverine that 'goes back' as he is the only one who can survive the process with his auto healing ability.
Days of Future Past is a typical sci-fi laden romp, but in it's wanting to be something special, and united the two eras of the X-Men films it seems to get lost in itself in the story telling, and you have to take things at face (or screen) value.
The one thing I have always admired about the series is it's ability to bring all the actors back for another film, and although the older generation has little to do in this, and many of them are little more than a cameo, they are all present and correct.
An enjoyable film, but instantly forgettable, it's not one of the best X-Men films, with an time travel element that is a little hard to swallow, and a convenient ending that simply seems to reset the universe ready for the already announced sequel.
If your a fan of the series go see it and enjoy every moment, if not you may fin it a little hard to swallow.