When numerous mysterious murders take place in Seattle, Edward suspects that they might have been committed by a group of newly-born vampires, who are incapable of controlling themselves.
Bella wants to see Jacob, her werewolf-friend. He tells Edward about it. Meanwhile Alice has a vision in which Victoria comes to the town to take revenge for James'' death, who was her lover and was killed by Edward. Now Victoria wants to do the same to him - she intends to kill his beloved, Bella. Edward worries about his girlfriend''s safety, but she assures him that she will be safe with Jacob and his werewolf family. Eventually she persuades Edward who lets her meet with Jacob regularly.
Bella wants to have sex with Edward before being changed into a vampire. He refuses firmly, because in such a situation he couldn''t control himself and he could kill his partner. Seeing, though, how important it is for Bella, he promises her to try in the future, but only if they marry first. Because Bella''s parents divorced, her vision of marriage isn''t optimistic. But she agrees, because she wants to be with Edward.
It becomes clear that murders in Seattle have been committed by an army of newborns, who are led by Victoria. The Cullens and werewolves decide to fight together. They are trained by Jasper to prepare for the battle. Edward doesn''t participate in it directly, he stays with Bella in the mountains. Seth and hurt Jacob join them. When Jacob hears in the morning Edward and Bella''s conversation about their engagement, he''s devastated. He also loves Bella. His life became pointless when she finally chose Edward. To refrain Jacob from trying to kill himself, Bella kisses him and tells him that she also loves him. Edward has to fight with Victoria.
Victoria and her army are defeated. Bella has to choose between Edward and Jacob. She tells Jacob that she loves him, but she also admits that she loves Edward more. When Jacob receives an invitation to Edward and Bella''s wedding, he turns into a werewolf and runs away because of the pain caused by Bella''s decision to become a vampire.
Summaries of the other parts of the cycle:
1) "Twilight": http://www.shvoong.com/books/horror/1870757-twilight/
2) "New Moon": http://www.shvoong.com/books/horror/1870978-new-moon-second-twilight-series/