Jimmy (Mathew Horne) and Fletch (James Corden) are two friends living in London experiencing life problems. Jimmy is dumped by his girlfriend and Fletch is fired from his job as a clown. They decide to escape their woes and hike to a remote village in Norfolk they find on an old map. As they arrive at a pub in the village, with Jimmy upset about Fletch destroying his phone, they see a number of attractive foreign female history students leaving.
Hoping to find more beautiful women inside, they are greeted by a morose crowd of men and approached by a seemingly crazed vicar (Paul McGann) who believes Jimmy is a long lost descendant of a local vampire slayer. As the barman offers the two men free ale as an apology for the vicar, they learn the students they saw earlier are going to a cottage - where they are to stay the night. Jimmy and Fletch pursue the students' bus, catching up to it as the engine has broken down, and are introduced to four girls (Heidi, Lotte, Anke and Trudi). They are invited to join a party on the bus.
The group arrives at their destination, only to learn that a curse rests over the village and that every female child turns into a lesbian vampire on her eighteenth birthday. There is an old legend stating that the Vampire Queen, Carmilla, descended on the village, killed its menfolk and seduced its women to her evil. When the ruler of the land, Baron Wolfgang Mclaren (Jimmy's great ancestor) returned from the Crusades, he discovered one of the women corrupted by Carmilla was his wife, Eva. The baron forged a sacred sword, then defeated Carmilla, but before dying, Carmilla cursed the village, adding that when the blood of the last of Mclaren's bloodline mixed with a virgin girl's blood, Carmilla would be resurrected.
Fletch and Jimmy spend the night with the women. Heidi, Anke and Trudi are turned into vampires. After Lotte insists that the others try to find her missing friends, they witness Trudi being turned. Eva, Carmilla's mistress, tries to draw Lotte to her growing clan of lesbian vampires. The trio runs back into the cottage after killing vampires Heidi and Anke and barricade themselves in after the vampires destroy the bus. Jimmy's ex-girlfriend Judi arrives at the door and Jimmy, not ready to give up on the relationship, takes her into the bedroom. Lotte reveals to Fletch that she is a virgin and wants to sleep with Jimmy.
At the church, the Vicar researches the vampire slayer who killed Carmilla before arming himself and setting off to find Jimmy. Judi reveals herself to be a vampire, and after a struggle, Fletch and Jimmy kill her. The vampires approach the cottage and Jimmy inadvertently invites them in. Eva discovers that Jimmy is the descendant of the baron who killed Carmilla and that Lotte is a virgin and kidnaps them.
The Vicar saves Fletch from Trudi and tells Fletch the truth about the village and Jimmy's identity. They go after Jimmy and Lotte in the Vicar's crucifix-covered car. As the vampires prepare to sacrifice Lotte and Jimmy, Fletch and the Vicar try to recover the sword of Daeldo, the sword that killed Carmilla, from the baron's tomb. While Fletch works to open the tomb, the Vicar checks on his daughter Rebecca, but does not notice that she has been turned. Rebecca attempts to seduce Fletch, who does not know what she is. When she attacks him, she is inadvertently impaled on the sword. Fletch decides not to tell the Vicar of his daughter's death.
At Carmilla's tomb, Lotte reveals her love for Jimmy. The vampires begin draining the two of their blood to resurrect Carmilla. With the sword, Fletch and the Vicar drive to Carmilla's tomb. When they enter the woods, they bring various weapons, but forget the sword. Despite not having the sword, the pair reach Jimmy and Lotte. The Vicar releases them, but not before enough blood gathers to resurrect Carmilla. The Vicar sacrifices himself so the others can get back to the car and the sword. Eva separates Lotte from the men, attacking and seducing her. Lotte fights back while Fletch and Jimmy fetch weapons. Lotte kills Eva with her cross necklace, infuriating Carmilla. Fletch tries to kill Carmilla before Lotte is turned, but is captured himself. Jimmy saves them by hurling the sword at Carmilla, piercing her heart and destroying her for good. With the curse lifted, the three survivors decide to continue ridding the world of evil.
The film ends with the shot of a "Gay Werewolf" howling before the full moon.