As Seattle is ravaged by a string of mysterious killings and a malicious vampire continues her quest for revenge, Bella once again finds herself surrounded by danger. In the midst of it all, she is forced to choose between her love for Edward and her friendship with Jacobknowing that her decision has the potential to ignite the ageless struggle between vampire and werewolf. Can Bella choose between her best friend and the love of her life?
The book begins with a note from Jacob to Bella. It explains that he misses her as well, but that this changes nothing between them. Charlie then 'un-grounds' Bella, but on the condition that she spend more time with her friends, especially Jacob, and not just Edward. Meanwhile, Alice has a vision that Victoria, the vampire that is hunting her, is back in town, so Edward, keeping this knowledge from Bella, talks Charlie into letting them take a weekend trip to Florida to see Bella's mother. When they return, Jacob confronts Edward about keeping Victoria's return a secret from Bella. Jacob and Bella reconcile, and she starts spending more time with him.
Eventually, Edward tells Bella what happened: Alice had a vision of Victoria coming back, so Edward, as a precaution, convinced Bella to take the trip to Florida. While they were there, Victoria came looking for Bella, and both the Cullens and the Werewolves hunt her. However, there is a minor altercation between the two groups during the chase, and Victoria slips away. From this point in the novel, Edward is displeased that Bella is spending time with Jacob; in fact, while he is away hunting, he even bribes Alice with a new car to keep Bella away from the werewolves. This doesn't work, however, and Bella slips away. Bella insists that Jacob and the rest of the pack are safe and that Jacob would never harm her. Edward finally agrees with her and willingly takes her, on occasion, to the border (the area where the pact between the Quileutes and the Cullens was made), where Jacob picks her up and takes her to his house. During one of these visits, Jacob clarifies his emotions toward Bella; he reveals that he loves her and forcefully kisses her, much to her displeasure. She reacts by punching him, injuring her hand, and not him.
Edward threatens Jacob when Bella is brought back home. Afterwards they make a treaty and promise to work together. Later, Alice convinces Charlie to let Bella spend the weekend at her house because the rest of the family has gone camping and she doesn't want to be alone; this is really a lie, as Alice and the other Cullens plan to go hunting over the weekend, leaving Edward and Bella alone at the Cullens' house. Edward proposes again, and Bella accepts under the condition that that Edward turns her into a vampire. Bella expresses the worry of what other people will think if she gets married so early, and is reluctant to the marriage and engagement as a whole; she refuses to wear the engagement ring, which had belonged to Edward's mother.
Bella discovers that a vampire has taken things from her room for the purpose of using her scent to locate her. Meanwhile, the Cullens realize that there is a coven of newborn vampires committing the murders in Seattle. Bella soon makes the connection between both incidents, and realizes that the new vampires are being led by Victoria. She reports this to the Cullens, who then decide to join with the Quileutes and capture and kill the newborns. Alice foresees a battle between Victoria's coven and the joined forces of the werewolves and the Cullens.
Edward and Bella camp up in the mountains, where they are joined by Jacob and Seth- a younger member of the pack- in wait of the war. Bella and Edward use Bella's scent to set a trap for the newborns to follow, which will lead them to face the Cullens and the werewolves. Bella succumbs to the chilly climate, and Jacob, with the intention of taunting Edward, slips into Bella's sleeping bag in order to keep her warm. Feigning sleep, she witnesses a conversation between Edward and Jacob, in which Jacob openly confesses his feelings for Bella to Edward. And Edward admits that Jacob can give Bella some things that he, Edward, can't. The two of them come to an understanding.
In the morning, Edward and Bella begin talking about their engagement; they hear a howl of pain, and Bella realize that it is Jacob, who had been listening in to their conversation. He runs away before Bella can reach him, and so she asks Edward to go after him. When they come back, Edward gives them some space to talk. Jacob threatens to join the fight and get himself killed; when Bella begs him not to, he insists that she ask him to kiss her so he won't kill himself (later on, it is revealed that he had tricked her into believing he would). She does so, and she realizes that she loves Jacob, but is not in love with him the same way she is with Edward.
When Edward returns, he admits that he overheard everything and that he's not angry at her for loving- and kissing- Jacob; this makes Bella feel guilty. As the two stay at the camp with Seth, Victoria attacks with her newborns. She soon arrives to confront Bella and Edward, along with one of her newborns, Riley. A fight breaks out, and in the end Seth, in his wolf form, kills Riley as Edward destroys Victoria.
The Cullens, as well as the pack, are unharmed during the fight; but it turns out that Jacob was injured. Carlisle assures Bella that he is healing and that no permanent damage was done. The Volturi arrive to clean up the war, but they are too late. They remind Edward of the agreement they had to change Bella into a vampire, to which Alice replies that the date has been set.
Bella goes to visit Jacob, who is on the mend, a while later; they admit that they love each other, but Bella insists that she cannot live without Edward. However, Jacob tells her that if she ever changes her mind, he will always be there for her.
Edward places the engagement ring back on Bella's finger, as she no longer fears what others think. Bella agrees to let Alice plan her wedding and asks her to be the maid of honor. She also tells Edward that it is time for them to tell Charlie about the engagement.
The epilogue is from Jacob's perspective:
He receives an invitation to Bella and Edward's wedding, and feels angry now that it is clear Bella will go through with "killing herself." He also finds a letter from Edward within the invitation; Edward tells Jacob that he doesn't have to come to the wedding if he doesn't want to, and thanks him for all he's done for Bella. The letter only angers Jacob further. Ignoring the questions and pleas of the pack, he changes into his wolf form and takes off, wanting nothing more than to "leave Jacob Black behind." He decides that he is no longer hurt that Bella would choose Edward over him; however, he questions how things would be between them once Bella becomes a vampire.