In an unnamed American town, teenagers Darren Shan and Steve Leonard, Darren’s more rebellious best friend and classmate, sneak out at night to attend the Cirque du Freak freak show at a local theater. During the performance, Steve recognizes one of the troupe’s members, Larten Crepsley, as a vampire who is over a hundred years old, prompting him to try to find Crepsley after the show. Meanwhile, Darren is fascinated by Crepsley’s spider, Madam Octa. He goes backstage and impulsively steals her, but is forced to hide as Crepsley returns to his green room. Steve arrives, begging to be made into a vampire, but Crepsley refuses after tasting Steve’s blood, which he says “tastes of evil.” When Crepsley realizes that Madam Octa has been stolen, Darren flees from the theater with Crepsley hot on his trail. He escapes when the mysterious Mr. Tiny arrives in his limousine, accompanied by Murlough. The latter is unimpressed with Darren, dismissing him as a “bag of blood,” but Tiny promises to stay in touch after dropping Darren off at his home.
The following day, Darren secretly brings Madam Octa to school, where she escapes. After a struggle in a crowded hallway, Madam Octa bites Steve on the cheek, leaving him to succumb to her deadly venom in the hospital. Darren returns to the theater and begs Crepsley for help. Initially annoyed and contemptuous, Crepsley agrees on the condition that Darren becomes a half-vampire and Crepsley’s personal assistant. Darren agrees, only to flee while Crepsley administers an antidote to Steve. However, after nearly attacking his own sister out of vampiric bloodlust, Darren decides to leave with Crepsley, who uses magic to stage Darren’s death in a fall from a roof.
After being buried in a cemetery, Darren is dug up by Crepsley, but Murlough ambushes them. Crepsley fights him off and the two go to the Cirque du Freak campgrounds, where Darren meets Evra Von, the snake boy, and Rebecca, the monkey girl. Meanwhile, Steve, contemplating suicide after losing his best friend, is stopped by Tiny, who offers him a chance to become a Vampaneze, a race of vampires who (unlike Crepsley and others) murder their victims to feed on their blood. Steve agrees, proceeding to kill high school history teacher Mr. Kersey with assistance from Murlough. Trying to instigate a confrontation between Steve and Darren, Tiny kidnaps Rebecca and Darren’s family, leaving a flyer for the Cirque du Freak at Darren’s former home.