On her 18th birthday, Bella Swan wakes up from a dream in which she sees herself as an old woman. She expresses her distaste with growing older than her boyfriend Edward Cullen, a vampire who stopped aging physically at 17. Despite her lack of enthusiasm, Edward's adoptive family throws Bella a birthday party. While unwrapping a gift, Bella gets a paper cut, causing Edward's brother, Jasper, to become overwhelmed by her blood's scent and attempt to kill her. Realizing the danger that he and his family pose to Bella, Edward ends their relationship, and the Cullens leave Forks, Washington.
Edward's departure leaves Bella heartbroken and depressed for months; however, when her father, Charlie, finally decides to send her to live with her mother in Florida, Bella refuses and agrees to spend more time with her friends. After seeing a movie with Jessica, Bella sees a group of men on motorcycles. This reminds her of when Edward previously rescued her from an assault, and she sees his image warning her to stay away. Bella discovers that all thrill-seeking activities evoke Edward's preserved image. She is also comforted by her deepening friendship with Jacob Black, a cheerful companion who eases her pain over losing Edward. When Jacob suddenly begins avoiding her, Bella discovers he has become a werewolf, an age-old enemy of vampires. Jacob's pack members are on constant patrol for Victoria, a vampire who wants to kill Bella to avenge the death of her mate by Edward's hand, leaving Jacob little time to spend with her. Alone again, Bella returns to seeking thrill-inducing activities.
Through a series of miscommunications, Edward believes Bella has killed herself by jumping off a cliff into the ocean. Distraught over her supposed suicide, Edward travels to Italy to provoke the Volturi—a powerful vampire coven capable of killing him—by exposing himself as a vampire to humans. Alice, Edward's sister, and Bella rush to Italy to save Edward, and arrive just in time to stop him. It is then that Edward tells Bella that he always loved her and only left to protect her. However, the Volturi determine that Bella, a human who knows that vampires exist, must either be killed or transformed into a vampire herself. Alice stops them from killing her by sharing her premonition with Aro (Michael Sheen)—a Volturi elder who is able to read thoughts through touch—in which Bella has been transformed. Soon after, they return to Forks and Bella forgives Edward for leaving her. The Cullens vote in favor of Bella being transformed into a vampire, much to Edward and Rosalie's dismay. Later on, Jacob reminds Edward of the treaty the Cullens made with the Quileute tribe: they will not attack each other, as long as the Cullens do not bite any humans—an action necessary for Bella's transformation. The movie concludes with Edward telling Bella that he will change her into a vampire, but only if she marries him first.