Two American college students, David Kessler and Jack Goodman, backpack across the North York Moors. As darkness falls, they stop for the night at a pub called "The Slaughtered Lamb". Jack notices a five-pointed star on the wall. When Goodman asks about it, the pubgoers stop talking and become hostile. The pair decides to leave, although the pub landlady insists they "can't let them go". Instead of changing their minds, the local clients only warn them to keep to the road, stay clear of the moors and beware of the full moon. While talking, David and Jack end up wandering off the road onto the moors. Jack and David hear sinister howls, which seem to be getting closer. They start back to the Slaughtered Lamb but realize that they are now lost. The boys are attacked by a supernaturally large wolf-like animal and Jack is killed. The attacker is shot by some of the pubgoers but instead of a dead animal, David sees the corpse of a naked man lying next to him. David survives the mauling and is taken to a hospital in London.
When David wakes up three weeks later, he does not remember what happened. He is interviewed by police Inspector Villiers who tells him that he and Jack were attacked by an escaped lunatic. David insists that they were actually attacked by a large dog or wolf. Jack appears to David as a reanimated corpse to explain that they were attacked by a werewolf, and that David is now a werewolf. Jack urges David to kill himself before the next full moon, not only because Jack is cursed to exist in a state of living death for as long as the bloodline of the werewolf that attacked them survives, but also to prevent David from inflicting the same fate on anyone else. Meanwhile, Dr. Hirsch takes a trip to the Slaughtered Lamb to see if what David has told him is true. When asked about the incident, the pubgoers deny any knowledge of David, Jack, or their attacker. But one distraught pubgoer speaks to Dr. Hirsch outside the pub and says that David should not have been taken away, and that he and everyone else will be in danger when he changes, only to be quickly silenced by a fellow pubgoer.
Upon his release from the hospital, David moves in with Alex Price, a pretty young nurse who grew infatuated with him in the hospital. He stays in Alex's London apartment, where they later make love for the first time. Jack, in an advanced stage of decay, appears to David to warn him that he will turn into a werewolf the next day. Jack again advises David to take his own life to avoid killing innocent people. When the full moon rises, David painfully transforms into a werewolf and starts to prowl the streets and the London Underground, slaughtering six Londoners in the process. When he wakes in the morning, he is naked on the floor of the wolf cage at London Zoo, has no memory of his activities, and is unharmed by the resident wolves.
David realizes that Jack was right about everything and that he is responsible for the murders of the night before. After failing to get himself arrested, David tries to slit his wrists with a pocket knife. David sees Jack, in a yet more advanced stage of decay, outside an adult cinema in Piccadilly Circus. Inside, Jack is accompanied by David's victims from the previous night, all of whom are furious at David for killing them. They all then insist that he must commit suicide before turning into a werewolf again. While talking with them as they try to offer him the least painful way to kill himself, David transforms and goes on another killing spree. After bursting out of the cinema, biting off Inspector Villiers' head in the process, David wreaks havoc in the streets and is ultimately cornered in an alley by police. Alex tries to calm him down by telling him that she loves him. Though he is apparently placated for a moment, he is shot and killed when he lunges forward, returning to human form in front of a grieving Alex as he dies.
Output: Jack warns David to kill himself before what event?