Protagonist Isabella "Bella" Swan is thrown an eighteenth birthday party by Alice and Edward Cullen, the vampire she loves, and the rest of their vampire family. While unwrapping a gift, she gets a paper cut. Edward's brother Jasper, although attempting a non-human diet, is overwhelmed by the scent of blood and tries to attack Bella. In an attempt to keep her safe from the world of vampires, Edward tells Bella that he doesn't want her anymore and leaves the town of Forks, Washington, with his family. Bella becomes severely depressed and seeks comfort with Jacob Black, an old family friend who eases her pain over losing Edward.Bella and Jacob begin spending a lot of time together, and Bella soon discovers that the rush of adrenaline present when she places herself in dangerous situations stimulates hallucinations of Edward's voice. Bella begins seeking out dangerous behavior, such as riding a motorcycle, to trigger the illusions. Meanwhile, Bella finds out that Jacob is a werewolf. The werewolves begin to protect Bella from the pursuit of the vampire Victoria, who seeks revenge for the death of her mate, James, who was killed by the Cullens in Twilight. After Bella attempts cliff-diving near La Push, she gets caught in a riptide and is rescued by Jacob.
At the same time, Alice Cullen, Edward's vampire sister who has visions of the future, has a vision of Bella jumping off the cliff. Presuming that Bella is dead, Alice rushes to Forks to check on Charlie Swan, Bella's father, while Edward calls Bella's home. Jacob answers and informs Edward that Charlie is at a funeral, referring to that of a man in town who had died of a heart attack, but Edward draws the conclusion that Jacob means Bella's funeral. Desperate after Bella's supposed death, Edward flies to Italy to see the Volturi, peace-keeping vampires who would be able to kill Edward, granting him leave of a world without Bella.
Bella and Alice rush to Italy to stop Edward, and save him before it is too late. Before they leave Italy, the Volturi tell them that Bella, a human who knows of the existence of vampires, must either be killed or changed into a vampire herself. After they return to Forks, Edward explains to Bella that he only left in order to protect her, and she forgives him. The book ends with the Cullens voting in favor of Bella being changed into a vampire after her graduation, much to Edward's dismay.