On Isabella Swan's (who prefers to be called Bella) 18th birthday, Edward Cullen, the vampire she loves, and his family throw her a birthday party. While unwrapping a gift, she gets a paper cut, which causes Edward's adopted brother [Jasper], to be overwhelmed by her blood's scent and he attempts to kill Bella. Trying to protect her, Edward and the Cullens move away from [Forks], but in an attempt to get Bella to move on, Edward tells her it is because he no longer loves her. This leaves Bella heartbroken and depressed.
In the months that follow, Bella learns that thrill-seeking activities, such as motorcycle riding and cliff-diving, allow her to "hear" Edward's voice in her head. She also seeks comfort in her deepening friendship with Jacob Black, a cheerful companion who eases her pain over losing Edward. Sometime after losing Edward, Bella starts to enjoy Jacob's company and friendship. After spending some time with Bella, Jacob starts experiencing some huge, unexpected and drastic changes with his mood swings, body and personality. As Jacob undergoes a very long, painful and life altering transformation Bella and Charlie become concerned. A few weeks later, Bella notes that Jacob has changed and that he isn't as happy-go-lucky as he once was. She isn't so comfortable with the new Jacob and shortly thereafter she discovers that Jacob has unwillingly become a werewolf and that there are other tribe members who are werewolves too. Jacob and his pack protect Bella from the vampire Laurent and also Victoria, who seeks revenge for her dead mate, James, whom the Cullens had killed (in Twilight).
Meanwhile, a series of miscommunication leads Edward to believe that Bella has killed herself by jumping off a cliff. Distraught over her supposed suicide, Edward flees to Volterra, Italy to provoke the Volturi, vampire royalty who are capable of killing him. Alice and Bella rush to Italy to save Edward, arriving just in time to stop him. Before leaving Italy, the Volturi tell Edward that Bella, a human who knows that vampires exist, must either be killed or transformed into a vampire to protect the secret. When they return to Forks, Edward tells Bella that he has always loved her and only left Forks to protect her. She forgives him, and the Cullens vote in favor of Bella being transformed into a vampire, to Rose and Edward's dismay. However, Jacob sternly reminds Edward about an important piece in their treaty: if the Cullens bite a human, the treaty is over and the wolves will attack. When Bella reminds him that it's none of his concern as being a vampire is what she wants, Jacob reveals it is his business as she doesn't understand what's going to be at stake for her and the Cullens. Before he can continue warning her, they hear Charlie screaming at Bella to get inside the house at once. Jacob apologizes to Bella once more before leaving and the book ends with Charlie grounding her.