Becca Crane (Jenn Proske) moves to Sporks to live with her clueless father, Sheriff Frank (Diedrich Bader), after her mother starts an affair with Tiger Woods. Meanwhile, a killing spree targeting random locals is attributed to the Canadians, but the real perpetrators are a group of vampires usually mistaken for the Black Eyed Peas. Becca quickly befriends many students at her new high school, including Jennifer (Anneliese van der Pol). She is also intrigued by the mysterious and aloof Edward Sullen (Matt Lanter), who perplexes her during their time in the biology class, due his odd behavior.
Later, when Becca is nearly struck by a van in the school parking lot, Edward stops the vehicle by thrusting another student in the van's way. Becca later "dreams" Edward was on her room; in truth, he is but is repulsed by things she does sleeping. After much research and thought, Becca confronts Edward and tells him she thinks that he's a Jonas Brother. Edward corrects her, saying he is a vampire but that he only consumes animal blood. Despite the danger of being around a vampire, Becca agrees to go to prom with Edward. Later, Becca and Edward kiss passionately in her room; Becca attempts to seduce Edward into having sex, but he abstains.
On Becca's birthday, Edward introduces Becca to his vampire family. While unwrapping a gift, Becca gets a paper cut; Edward's brother Jeremiah attempts to bite her, but is knocked away. To keep his family away from Becca, Ed distracts them and takes her out to the woods. He then proceeds to break up with Becca, who throws a tantrum after he leaves. Much to her expectance, Becca is attacked by three nomadic vampires, but Edward intervenes and saves her.
Edward's departure leaves Becca heartbroken for months, but she is comforted by her deepening friendship with her childhood friend Jacob White (Christopher N. Riggi). When Becca is accosted in the woods by the nomadic vampires again, Jacob transforms into a Chihuahua as his werewolf pack arrives to save her. Meanwhile, Edward has moved to Rio and is now dating Lady Gaga to get over losing Becca. He is later informed by his sister Isis, via her gift of prophecy, that Becca killed herself. Edward becomes depressed and decides to provoke the "Zolturi", a powerful and narcissistic vampire coven, into killing him by exposing himself in the sunlight in front of humans, thus exposing the existence of vampires. Isis has another vision of Becca's survival as he leaves, but she is unable to warn him.
Isis arrives at Becca's house, and tells her she has to save Edward by showing him she is still alive. To Becca's shock, the Zolturi are currently partying at the prom due to the St. Savadore theme. Jacob appears and demands that Becca choose between him and Edward, but just before she announces her decision, he is distracted by a cat and runs off to chase it. Upon arriving at the prom, Becca is caught between the warring factions of Edward fangirls and Jacob fangirls. She is unable to reach Edward before he exposes himself, figuratively and literally. However, twilight occurs, followed by a new moon and an eclipse, concealing Edward's vampiric nature as Becca gets him to safety. Frank arrives to check on Becca, making her hopeful; however, he thinks supernatural creatures are the prom theme and leaves. After a fight between him and the Zolturi leader, Daro (Ken Jeong), Edward is forced to turn Becca into a vampire or he will be killed horribly. He agrees to do so only on the condition that she marry him, which she accepts.
The film concludes with Edward being struck in the head by the Jacob fangirls' leader. Edward survives the blow, and the girl is attacked by the newly vampirized Becca.