David Kessler and Jack Goodman, two American friends backpacking across Northern England, hitch a ride on a truck transporting a flock of sheep across the moors and disembark at a fork in the road. Before driving away, the truck driver warns, “Keep off the moors. Stick to the roads.” David and Jack walk to the village of East Proctor and happen upon a pub called The Slaughtered Lamb. Inside the pub, Jack notices a pentangle drawn on one wall of the pub and explains to David that the pentangle is used in witchcraft and is supposedly the “mark of the wolfman.” When Jack asks the local pub-goers about the pentangle, he and David receive a cold response and decide to leave. The bartender insists they stay, but the other locals tell them to go and warn them to “Stay on the road, keep clear of the moors” and “beware the moon.” As Jack and David walk in the rain, the locals inside the pub argue amongst themselves as to the safety of letting the two Americans leave. When they hear the cry of a wolf, the bartender insists the locals go find David and Jack and bring them back to the pub. Outside, David and Jack also hear the noise and notice there is a full moon. Realizing they have wandered off the road, they turn to find their way back to The Slaughtered Lamb. David falls in the darkness, and as Jack reaches to help him up, an animal attacks Jack. David runs away but eventually turns back to help Jack. The animal then attacks David but is stopped by gunshots fired from the locals from The Slaughtered Lamb. David looks to his side, where he sees the body of a naked man riddled with bullet holes. Three weeks later, David lies unconscious in a hospital in London, where he has been since the attack. As Nurse Alex Price attends to him, David cries out for Jack in his sleep. Doctor Hirsch enters the room and tells Alex that David was attacked by an escaped lunatic. Still unconscious, David dreams of running through the woods. When David awakens, Dr. Hirsch informs him that Jack was killed by an escaped lunatic who attacked them on the moors. David argues that their attacker was a wolf, not a man. Later, two detectives from Scotland Yard speak to David, but they do not listen when he repeats his belief that the attacker was not human. David suffers from progressively more disturbing and graphic nightmares, including one in which he attacks and eats a deer with his bare hands. One morning, David sees an apparition of Jack, bloody and mangled from the attack. Jack tells David they were attacked by a werewolf and that he will be stuck in limbo, a member of the “undead,” until the last of the werewolf’s bloodline is killed. Since David was bitten and survived, he is now a werewolf and the last of his kind. Jack tells David he must kill himself to save the lives of others and warns him to “beware the moon.” Later, Alex checks on David. He kisses her, and tells her, “I’m a werewolf,” though she suggests his conversation with Jack was a dream. When David is discharged from the hospital, Alex invites him to stay at her flat. Once there, she tells David she finds him attractive, and they make love. When David awakens, he goes to the bathroom, where he sees Jack, who is now more decomposed. Jack warns David that tomorrow he will transform into a werewolf when the full moon rises and insists David kill himself before that happens. Alex finds David and takes him back to bed, where he tells her the plot of the film The Wolfman , starring Bela Lugosi. Based on the film, David believes that a werewolf can only be killed by a loved one. Later, intrigued by David’s story, Hirsch travels to East Proctor to investigate. He goes to The Slaughtered Lamb and, after speaking with the locals, becomes convinced they are hiding the truth about the attack. David accidentally locks himself out of Alex’s flat after she leaves for work. Outside, he notices animals acting strangely towards him, including a small dog that barks viciously and a cat that hisses. David manages to get back inside, and, when the full moon rises, David transforms into a werewolf. Outside of Alex’s apartment, a couple exits a cab and walks to the back of the building. David, now fully transformed into a werewolf, attacks the couple. At the hospital, Hirsch reveals to Alex that he is concerned about David’s welfare, especially considering there is a full moon. He tells Alex about his visit to East Proctor and that he believes David might harm himself or others during his werewolf “fantasies.” Meanwhile, David, as the werewolf, attacks and kills three homeless men in a junkyard, and, later, a man in a subway station. The next morning, David awakens to find himself nude in the wolf pen at the zoo. He steals a child’s balloons and a woman’s coat to cover himself and runs back to Alex’s flat. Elsewhere, Hirsch reads a newspaper article describing six gruesome murders committed the night before. When David arrives at Alex’s flat, he tells her he does not remember any details from the night before. Hirsch telephones Alex and tells her to bring David to the hospital immediately. It is only when they are en route to the hospital that they learn of last night’s murders. Convinced he committed the murders, David tries to turn himself in to a police officer, but the officer does not believe him. Afraid of his potential for violence, David professes his love for Alex, then runs away into the bustling city. David calls his family from a phone booth in Piccadilly Circus and asks his sister to tell his parents and younger brother he loves them. David hangs up the phone and tries to slit his wrist with a pocketknife, but he does not have the courage. Outside the booth, he sees Jack on the other side of the street. David follows the badly decomposed Jack into a nearby movie theater sparsely populated by men watching a pornographic film. Jack evokes the apparitions of the six people David murdered the night before, who each suggest ways for David to kill himself. Jack stresses that none of the victims will be at peace until David dies. As the moon comes out, David transforms into a werewolf and kills the patrons in the theater. Police attempt to seal off the entrance to the movie theater, but David breaks out, causing chaos as he runs through the streets. Back at the hospital, Hirsch and Alex hear of the attacks and go to Piccadilly Circus. Police corner David on a dead end street. When Alex and Hirsch arrive, Alex breaks through the police line and runs to David. Alex tells David she loves him, but when he moves to attack her, the police open fire, killing David on the spot. Alex weeps as David, now in human form, lies dead.