Bella Swan dreads waking up on her birthday because she is turning 18. The milestone is just another reminder that she keeps aging while her immortal vampire boyfriend, Edward Cullen, remains eternally 17. Despite her begging, he repeatedly refuses to change her into a vampire like him. He fears in doing so, he would be damning her soul.
The Cullen "family," a coven of seven vampires living in Bella's small town of Forks, Wash., throws a birthday party for Bella. As she tears open a present, she gets a bleeding paper cut. Though the Cullens are civilized vampires, who solemnly vow not to feast on people, they're still tantalized by the scent of human blood. Jasper, the most vulnerable of the clan, instinctually lunges at Bella. Edward saves her, but the attack is a jarring reminder to him that their relationship repeatedly puts Bella in mortal danger. A few days later, he tells her he no longer wants her and that he and his family are leaving Forks forever.
Bella becomes an empty shell of a person in the months that follow. She finally goes out with a girlfriend to placate her worried father, Charlie. Bella sees a group of suspicious-looking men on the same dark street where Edward once saved her and feels a sudden rush of adrenaline. She thinks she hears Edward's voice warning her to be careful. Suddenly, Bella feels alive again. She theorizes she can get Edward back, at least through hallucinations, if she can find ways to recreate that rush of adrenaline.
Bella sees two broken-down motorcycles for sale and remembers Charlie's warning about the dangers of riding them. Convinced riding motorcycles will produce the adrenaline rush she craves, Bella takes the bikes to her friend Jacob Black on the nearby La Push Indian reservation. Jacob, a master at vehicle repair, is slightly younger than Bella and smitten with her. He eagerly agrees to work on the bikes in secret, and Bella enjoys their time together. She makes it clear she doesn't share his romantic feelings, but she becomes desperate to hold on to his friendship.
Jacob and Bella ride the newly-repaired cycles several times. Although Bella gets her adrenaline rush — and consequently, hallucinations of Edward — she continues to crash and ends up in the ER. Afraid Charlie will start asking questions, she devises another way to "see" Edward. Bella takes Jacob on several hikes, secretly trying to find the clearing in which Edward first revealed himself to her as a vampire. She hopes to feel his presence there.
Without explanation, Jacob stops returning Bella's calls. This goes on for weeks. She hikes alone and finds Edward's clearing. There, she encounters an enemy vampire named Laurent. Bella first met Laurent (in Twilight) when he traveled with a vampire couple named James and Victoria. James stalked Bella, and Edward killed him. Now, Laurent warns her that Victoria is stalking Bella to avenge her mate's death. Just as Laurent decides to bite Bella himself, a pack of wolf-like creatures emerge from the forest and chases him away. Alone again, Bella wonders about the strange wolves and how they could terrify such a powerful vampire.
Increasingly anxious about Jacob, and about her own safety now that Victoria is near, Bella drives to La Push. The Jacob she finds there has larger muscles, shorter hair and intense, angry eyes. Although he explains little, Jacob's newfound fury is clearly directed at the Cullens. Jacob later sneaks into Bella's room. He's not allowed to tell her what's happened to him, so he urges her to remember a discussion they had months earlier. Bella remembers Jacob telling her stories of the "cold ones," which was how she learned Edward was a vampire. Then she recalls the rest of the legend: The Quileute Indians were the sworn enemies of vampires. Sometimes, when vampires became problematic in the area, members of the tribe involuntarily became werewolves.
Once Bella realizes Jacob and his friends have become werewolves, she shares what she knows to help them find and destroy Victoria. Bella spends a great deal of time at La Push, where the werewolves are better able to protect her. One day, on her ever-present quest to hear Edward in her imagination, Bella dives off a high cliff into the water. Jacob rescues her from the rushing current just in time. She begins to wonder if she could be with Jacob romantically, even though she could never love him like she loves Edward.
As Jacob brings her home from her near-drowning, she sees one of the Cullens' vehicles in her driveway. Jacob has a visceral reaction to the vampire smell, but Bella rushes into the house. Alice, Edward's sister, is relieved to find Bella alive. With her special powers, Alice had foreseen Bella's cliff dive and assumed she was suicidal. Alice stays a few days. She says Edward went off on his own after they left Forks, and she isn't sure where he is. Jacob confronts Bella and urges her to choose to be with him. As they talk, the phone rings. Jacob answers and responds harshly. The caller, whom Jacob senses is Edward's father, has asked for Charlie. Jacob responds, "He's at the funeral."
Charlie had gone to the funeral of a friend. The caller had been Edward. After hearing of Bella's cliff dive from another sister, Edward assumes Charlie is at Bella's funeral. Alice foresees Edward's mix-up. She also foresees his plans to go to Italy and have the Volturi — the most powerful vampire clan — end his existence. He has no desire to live if Bella is dead.
Alice and Bella act quickly, leaving Charlie a garbled note and catching a plane to Italy. Alice explains that Edward is planning to expose himself publicly as a vampire, which would endanger the Volturi's secret stronghold. The Volturi would have no choice but to destroy Edward. Alice foresees that Edward plans to show himself at noon in the middle of a festival. Once in Italy, the girls steal a car and race to the Volturi city. Bella rushes through the crowd, halting Edward in the nick of time. Their reunion is short-lived, as Volturi members usher Bella, Edward and Alice into their headquarters. They question and subtly threaten the trio, allowing them to go free only after Edward and Alice promise to turn Bella into a vampire soon.
After they've returned from Italy, Edward tells Bella he never stopped loving her and apologizes profusely for ending their relationship. Again, Bella says she wants to become a vampire, and Edward balks. Bella and Edward visit the Cullen home, where Bella asks the other family members to vote whether they want her to join them. Edward is out-voted, so he asks Bella to wait just a little longer. Then he asks her to marry him first. Bella hedges on the proposal, but the two promise to be together forever.
Charlie puts Bella under house arrest and remains skeptical of Edward. Bella ponders whether Victoria will endanger all of Bella's loved ones, if the Volturi will come after her or if Jacob will die trying to protect her. Ultimately, she believes she can face anything with Edward by her side once more.