Two teenagers unknowingly break a 100-year-old truce between two warring factions of vampires; vampires who do not harm their victims, and Vampaneze, who kill whomever they feed on.
Darren Shan and his best friend, Steve Leonard, go to a traveling freak show and Steve recognizes one of the performers as a vampire. Steve goes to see the vampire, Larten Crepsley, after the show and tries to convince him to turn him into a vampire. Crepsley insists that becoming a vampire means having to give up any hope of a normal life, and Gavner Purl notes that children aren’t supposed to become vampires anyway. Steve then reveals that he hates his life, and wants to be a vampire more than anything else. Crepsley reluctantly tests Steve’s blood, but quickly spits it out, saying he has bad blood. Steve furiously swears revenge on Crepsley and leaves. Darren, who is hiding in the cupboard after stealing Crepsley’s spider, Madam Octa, overhears the whole thing.
When Crepsley realizes that Madam Octa has been stolen, Darren flees from the theater with Crepsley hot on his trail. Outside there is a limo waiting for him, which he is told to get into. Inside he meets Desmond Tiny and mad Vampaneze Murlough. Murlough is unimpressed with Darren, but Mr. Tiny is certain he has great potential. They drop Darren off at his house where he sneaks back into his room.
The next day at school, Darren tries to take a look at Madam Octa, but Steve catches him and tries to get a close look at her while holding her cage. The bell rings, startling Steve and making him drop the caghim, and she escapes. As Madam Octa tries to escape the school she creates chaos and Steve chases her, trying to kill her, and Darren follows after trying to save her. At the last moment Madam Octa slips into Steve’s shirt, bites him, and makes good her escape. Madam Octa’s poisonous bite leaves Steve in the hospital, and it is likely he will not survive. Darren then goes to Crepsley to ask for the antidote and is offered a deal by him. Crepsley will keep Darren safe from Mr. Tiny and give him the antidote if Darren becomes a half vampire and Crepsley’s vampire assistant. They administer the antidote to Steve at the hospital, after which Darren, who still wishes to be rid of Crepsley, sounds the hospital alarm, forcing Crepsley to leave without him.
Even as a half-vampire, Darren finds continuing with a normal life too difficult. After nearly attacking his sister out of bloodlust, Darren agrees to leave his family and go to the Cirque Du Freak with Crepsley. He fakes his own death and a funeral is held for him. During the funeral, Steve notices the scars on Darren’s fingers and realizes he has become a vampire.
When Crepsley digs up Darren’s grave they are attacked by Murlough, who tries to kidnap Darren and make him into a vampaneze. They lose Murlough and return to the Cirque. Darren meets the snake boy, Evra Von, and Rebecca, the monkey girl. Meanwhile, Steve is contemplating suicide. Mr. Tiny stops him and offers him a chance to become a Vampaneze, like Murlough. Mr. Tiny tells Steve that he is neither a vampire nor a Vampaneze, but someone with an interest in both. Tiny learns Darren has not fed on humans yet and sends Vampaneze after him, knowing that he will be weak. Darren escapes, so they kidnap Rebecca and have Evra tell Darren to “come home”.
Meanwhile Mr. Tiny makes contact with Steve and entices him to join his Vampaneze by playing on his resentments and insecurities. Steve is turned into half-Vampaneze and then he and Murlough capture Darren’s family. Darren arrives home to find his family missing and a flier for Cirque du Freak. Darren arrives at the theater, where Murlough and Steve are waiting for him. They fight, but Darren is weak from not feeding. Crepsley arrives to fight Murlough. Rebecca frees herself, while Mr. Tiny watches from the balcony. Rebecca offers Darren some of her blood. He eventually agrees and fights with Steve using his new powers. Crepsley stabs Murlough and with his dying words Murlough declares that the truce between the two clans is broken. Steve and Darren continue to fight, but Tiny stops them and turns Murlough into a Little Person. Tiny and Steve then leave. Darren’s family are hypnotized to have forgotten what happened to them. Left by themselves, Rebecca and Darren then share a long heartfelt kiss before being interrupted by Mr. Crepsley. Crepsley gives Darren his own coffin, and Darren accepts his new life as a member of the Cirque du Freak.