On Isabella Swan's 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 attack and feed on 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 through her subconscious mind. 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 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) . Jacob starts developing physical emotions towards Bella but she doesn't feel the same having experienced a life changing breakup with Edward, making him horribly sad and envious of Edward. Jacob then saves Bella from drowning after jumping off the cliff, and almost kisses her in the events following.
Meanwhile, a series of miscommunications 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, though they refuse, deeming his mind reading ability to be too valuable. Alice Cullen, however, makes a surprise visit to Bella's house, which makes Bella overwhelmed. Bella asks a series of questions, and Alice tells her that she saw Bella trying to kill herself. Alice's vision changing about Edward, they feel out of place and rush to Italy to prevent Edward from revealing himself to humans so the Volturi are forced to kill him, 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 part of their treaty: if the Cullens bite a human for any reason, 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 asking Bella to get inside the house at once. Jacob apologizes to Bella once more before leaving and the book ends with Charlie grounding her for running off to Italy.