Alice comes back to Forks, thinking Bella is dead after seeing a vision of her throwing herself off a cliff. Finding her alive, she realises that the close presence of a werewolf (Jacob) is blocking her visions. Bella and Jacob are about to kiss when the phone rings - Jacob answers it and just says "he's not here - he's arranging a funeral". Hanging up, we discover it was Edward phoning to see if Alice's vision was correct - with Jacob's lie he thinks Bella is dead, and Alice sees a vision of him exposing himself as a vampire, which will cause the Volturi to kill him.
Alice and Bella rush to Italy, and stop Edward just in time, but still get the attention of the Volturi, who are fascinated by Bella's resistance to Edward's mind reading abilities. It turns out she's resistant to all vampire powers. Feeling her knowledge of the vampire world is a threat, they want to kill her or turn her, but Alice sees that she will eventually become a vampire, so they let her live.
Returning home, Bella asks the Cullens to vote on whether she can be turned, and the majority say yes. Edward asks her to wait, and to give him five years, which she negotiates down to three. Jacob is very upset when he discovers her willingness to become a vampire, and reminds Edward of a treaty between his werewolf clan and the Cullens, namely that they won't fight provided a Cullen never bites a human. Tempers rise and he turns into a wolf - the two are about to fight before Bella intervenes, and Jacob runs off. Edward says he'll turn her if she promises to be with him forever, and asks her to marry him.