It tells the story of a newborn vampire, Bree Tanner, who is featured in the third book of the series, Eclipse. The book is written from the viewpoint of Bree, as opposed to the rest of the series which is predominantly narrated by character Bella Swan. Meyer let director David Slade, screenwriter Melissa Rosenberg, and a few of the actors read a draft of the story during production of The Twilight Saga: Eclipse. The story begins with Bree and Diego hunting for human blood in Seattle, Washington. Bree has been a vampire for three months, and Diego has been one for eleven months. Together they kill and drink a pimp and two prostitutes. Bree and Diego discuss "her" (Victoria, who turned them into vampires). They hide in a cave and discuss their human lives, and how Riley came to offer them a second life as a vampire. Together they decide that Riley is using them as pawns, and that he might be lying to them. They also discover that sunlight does not kill a vampire, but makes their skin sparkle. They become friends and hunt for Riley and the other vampires they live with.

They find that Riley had relocated everyone to a log cabin and Diego gets into a fight. That night Bree and Diego stalk Riley, suspicious that he is meeting with "her." They eavesdrop on Riley's conversation with Victoria.

Eventually the Volturi show up, threatening to punish Victoria for amassing a vampire army, but are willing to give her army a chance to destroy the Cullen clan. The Volturi say that if Victoria does not attack within five days, they will kill her.

Bree returns to the log cabin and resolves to run away, while Diego stays behind to talk to Riley and inform him about the fact that the sunlight doesn't hurt them. Riley returns to the cabin alone and tells his vampire army that there are older vampires in Seattle (the Cullens) who want to kill them, and if they want to survive, they will have to work together and learn how to fight. Riley spots Bree and tells her that Diego is doing surveillance work with "her" and will return to join them in the fight. After three nights of training, Bree and the vampires hunt a ferry boat to drink the passengers' blood and regain their strength for the battle against the elder vampires. Riley then tells everyone that the vampires they will be fighting have yellow eyes and keep a human (Bella) as a pet; he gives them Bella's scent to hunt, describing her as the dessert.

They head off to fight the Cullens. Fred decides to run away to Vancouver before the battle, and agrees to wait for Bree for a day before leaving. Bree takes part in the battle in order to find Diego. But before the battle, Riley retreats, telling Bree that Diego has already started fighting with the group, and that he (Riley) has some other business. Bree arrives at the battle to find the newborn vampires being massacred by the Cullens, and thinks that Diego is already dead because she cannot see or smell him anywhere. She deduces that Victoria and Riley killed Diego for being disobedient the night he went missing.

She then turns to run away, but is cornered by Carlisle Cullen. They are joined by Jasper and Esme Cullen, who debate whether or not to kill her. They decide to restrain her until the Volturi arrive. Jasper forces her to close her eyes and presses his hands over her ears in order to protect the secret of the werewolves (howler vampires, as Bree calls them), because she can hear their howls.

Bree then spots Bella, and has trouble resisting the urge to drink Bella's blood (This scene is shown in Eclipse). The Volturi (dark-cloaks) then show up and Bree starts logically deducing facts about the Cullens, and their attitude towards the Volturi. She begins thinking about the conversation she and Diego eavesdropped on the night when Diego was killed. She guesses the redhead (Edward Cullen) to be the mindreader, and informs him about what Jane had told Victoria to do. Jane is dissatisfied with their explanations, and turns to interrogate Bree.

Jane then tortures Bree into telling everything about the newborns. Bree then pretends to act dumb, and hysterically states that Riley lied to her and everyone else, and they were ignorant of most things, and simply did as they were told, under fear of death. The Volturi decide to kill Bree, but Carlisle and Edward attempt to rescue her, by saying that they would take responsibility for her and teach her the rules. Bree can see that it is a futile attempt, and Jane confirms it by refusing to spare her life. Jane says that they do not give second chances, as it would adversely affect their reputation as law enforcers, and also warns them that Caius would display an interest in learning that Bella is still human, as he sees her as a threat to vampire secrecy. All of this is beyond Bree's comprehension, and she yearns for it to be over since Diego, for whom she had developed a romantic affection, is dead. Jane grants her wish by telling Felix to take care of her. Edward warns Bella to shut her eyes, but Bree believes that Edward is referring to herself. As she shuts her eyes, she is killed.