Bela and Edward now have a daughter, Renesmee, who is aging very quickly. The Volturi find out about her and it's against their law to create a child vampire because of something that happened in the past. But this kid isn't quite a vampire. So preparations of a war begins, with some vamps siding with the Cullens, others with the Volturi.
There's a huge, climatic battle between the good vampires and the Volturi--and many of the main characters die. Only it turns out to be a possible vision of the future, so no one dies and everyone lives happily ever after, which basically makes it a Disney movie. This is one of the worst Twilight films, tied with the second in terms of boredom.