The film starts with two young American men, David Kessler (played by Naughton) and Jack Goodman (played by Dunne), on a backpacking holiday in England. Following an awkwardly tense visit to a village pub, the two men venture deep into the moors at night. They are attacked by a werewolf, which results in Jack's death and David being taken to a London hospital. Through apparitions of his dead friend and disturbing dream sequences, David becomes informed that he is a werewolf and will transform at the next full moon.
Shooting took place mostly in London but also in Surrey and Wales. It was released in the United States on August 21, 1981 and grossed $30.56 million at the box office. Critics generally gave the film favourable reviews. The film won the 1981 Saturn Award for Best Horror Film and an Academy Award for Outstanding Achievement in Makeup. The film was one of three high-profile wolf-themed horror films released in 1981, alongside The Howling and Wolfen. Over the years, the film has accumulated a cult following and has been referred to as a cult classic.
The film was followed by a 1997 sequel, An American Werewolf in Paris, which featured a completely different cast and none of the original crew, and is distributed by Disney's Hollywood Pictures. A Hindi film Junoon was also inspired by this movie.
Empire magazine named An American Werewolf in London as the 107th greatest film of all time in September 2008.
Two American college students, David Kessler and Jack Goodman, are backpacking across the Yorkshire moors. As darkness falls, they decide to stop for the night at a pub called "The Slaughtered Lamb". Jack notices a five-pointed star on the wall. When he asks about it, the pub becomes very quiet and the pubgoers start acting very strange and hostile. The pair decide to leave, but not before the others offer them pieces of advice such as "Beware the moon, lads" and "Keep to the road." Whilst conversing with each other and wondering what they meant, they wander off the road, onto the moors.
Back at the pub, the owner gets very distressed and suggests that they go after the pair. As she says this, a sinister howling is heard. The rest of the pubgoers, having barricaded the door, decline. Back out on the moors, Jack and David have also heard the howls, and they seem to be steadily getting closer. They start back to the Slaughtered Lamb when they realize that they have left the road and are now lost on the moors. A full moon comes out from behind the clouds, and they remember the advice they were given earlier. The noises get steadily closer until they are stopped by a supernaturally large animal. The animal attacks both of them, and kills Jack. The animal is then shot and killed by the pubgoers, who have finally emerged. The beast changes into the dying body of a naked man. 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 and is told of his friend's death. David is questioned by the arrogant Inspector Villiers and the bumbling, but more understanding Sergeant McManus and learns that he and Jack were supposedly attacked by an escaped lunatic. David insists that they were actually attacked by a large wolf. But Villiers had already been told there were witnesses and an autopsy report of the maniac, so they deduce that David is suffering from shock.
David begins to have a series of bizarre nightmares. In the first, he runs through the woods, then decapitates and eats a deer. In the second, he is in a hospital bed with a monstrous, fanged face. In the final dream, he is at home with his family when they are attacked by Nazis with monstrous faces, wearing gas masks and wielding machine guns.
Things get stranger when Jack, now a reanimated corpse, comes to visit David and explains that they were attacked by a werewolf, stating that David himself is, in fact, 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 his eventual victims.
Trying to see if David is indeed telling the truth, his doctor, Hirsch takes a trip to the Slaughtered Lamb. 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. He's interrupted by another pubgoer, who remains silent. After more investigation, Dr. Hirsch finds out that the police report was "misplaced", and that David's wounds were cleaned and dressed before he was even looked at by the authorities. The doctor is convinced that the whole town was lying, and that David was indeed attacked by some sort of animal, though he is not convinced it was a werewolf.
Upon his release from the hospital, David moves in with Alex Price, the 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) suddenly appears to David again and tells him that he will turn into a werewolf the next day. Jack advises David to take his own life; otherwise he is doomed to kill innocent people who will then become the living dead.
When the full moon rises, as Jack had warned, David, who is alone in the apartment, begins to feel excruciating pain before stripping naked upon "burning up" and turning into a werewolf. He prowls the streets and the London Underground and slaughters six Londoners. When he wakes in the morning, he is naked on the floor of the wolf cage at London Zoo with no memory of his nocturnal, lupine, lycanthropic, carnivorous activities, but unharmed by the resident wolves.
Later that day, 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 and running from Alex, David calls his family in New York to basically say good-bye, though he only speaks to his little sister. Walking out of the phone booth after failing to slit his wrists with his pocket knife, David spots Jack (in a more advanced stage of decay) outside an adult cinema in Piccadilly Circus. Inside, Jack is accompanied by David's victims from the previous night. They all insist that he must commit suicide before turning into a werewolf again. While talking with them, night falls and, consequently, David turns into a werewolf again and goes on another killing spree. After busting out of the cinema (biting off Inspector Villiers' head in the process), a horrific melee ensues. David is ultimately cornered in an alley by the police. Alex arrives to calm him down by telling him that she loves him. Though he is apparently temporarily softened, he is shot and killed when he lunges forward, returning to human form in front of a grieving Alex as he dies.
- David Naughton as David Kessler
- Jenny Agutter as Nurse Alex Price
- Griffin Dunne as Jack Goodman
- John Woodvine as Dr. J.S. Hirsch
- Lila Kaye as Barmaid
- Frank Oz as Mr. Collins
- John Landis as Man smashed in window
- David Schofield as Dart Player
- Brian Glover as Chess Player
- Rik Mayall as Man in Pub (2nd Chess Player)
- Don McKillop as Inspector Villiers
- Paul Kember as Sergeant McManus
- Linzi Drew as Brenda Bristols
- Cameos and bit parts
In the Piccadilly Circus sequence, the man hit by a car and thrown through a store window is Landis himself.
As in most of the director's movies, Frank Oz makes an appearance: first as Mr. Collins from the American embassy in the hospital scene, and later as Miss Piggy in a dream sequence, when David's younger siblings watch a scene from The Muppet Show.
Actors in bit parts who were already—or would become—more well-known include the two chess players David and Jack meet in the pub, played by the familiar character actor Brian Glover and then-rising comedian and actor Rik Mayall. One of the policemen helping to chase and kill the werewolf is John Altman, who would later achieve fame as "Nasty" Nick Cotton in EastEnders. Alan Ford—later to appear in Lock, Stock and Two Smoking Barrels andSnatch—plays a taxi driver. The policeman in the cinema is played by John Salthouse and the policeman in Piccadilly Circus is played by Peter Ellis. Both Salthouse and Ellis appeared in police drama The Bill. David Schofield, known as Mercer from the Pirates of the Caribbean film series, plays the dart player at the Slaughtered Lamb and assists Dr. Hirsch in his investigation of David's attack.