A young woman from a rich Mexican family is under a curse that causes her to transform into a wolf-woman at night and kill people.

A psychic leads archaeologists into an Aztec pyramid, where they discover two mummies. One of these is a werewolf mummy. Both mummies revive and stalk the scientists. The werewolf mummy is played by Lon Chaney, Jr. in footage from La Casa del Terror, his last film role as a werewolf.

The film’s lack of story cohesion is what inspired a surrealist novel of the same title. Published in 2006, Face of the Screaming Werewolf by Ken Gage recycled imagery and dialogue from the film to tell a strange story involving Satan’s war in heaven, robots, sadomasochistic vampires, the nature of elitism, and aliens.

Wolves have been seen roaming around a girls’ boarding school, and when the girls begin to get murdered, suspicion focuses on both the wolves and on a newly-hired science teacher who might be a werewolf.

The story is set in 18th Century Spain. A beggar (Richard Wordsworth) arrives in the village of Santa Vera to find the streets deserted and the church bell tolling… and it is not a Sunday. He enquires at a bar and the locals explain that today is the wedding day of the Marques Siniestro and, by order, a public holiday. The villagers are unhappy because their taxes are paying for the wedding feast, but they are not invited. One man suggests that the beggar try his luck at the castle for “that’s where all our charity is!” The unfortunate beggar decides to take him at his word.

The Marques (Anthony Dawson) is a cruel man. When the new Marquesa (Josephine Llewellyn) is spooked by the sight of a roast goose, he humiliates the chef (Charles Lamb) in front of his guests. When a servant (Desmond Llewellyn) opens the door to the beggar, he warns him to leave. But it is too late: the Marques has already seen the beggar and invites him in. When Siniestro teases the man, his bride protests. So he offers to buy the beggar for her as a pet and gives the man ten pesetas. He then gets the beggar drunk and makes him dance for food. Finally, Siniestro makes him beg like a dog while his guests bark. Tiring of the joke, the Marques and his bride proceed to bed. But when the beggar wishes them a good night, Siniestro has him tossed into the dungeon for his ‘insolence’.

The imprisoned beggar is forgotten but manages to survive another fifteen years. His only human contact is with the jailer and his beautiful mute daughter (child: Loraine Carvana; adult: Yvonne Romain). Time has not been kind to the Marques either: his wife died and his friends deserted him, so that he has become a recluse in his room. When the jailer’s daughter, while cleaning in the Marques’ room, refuses the old man’s advances, he has her thrown into the dungeon with the beggar. The beggar, driven mad by his long confinement, rapes her and then dies.

The girl is released the next day and sent back up to see the Marques. But she has armed herself with a knife and kills the old man. She flees the castle and spends several months hiding in the forest living like an animal. She is found half-drowned in a pond by Don Alfredo Corledo (Clifford Evans) who lives alone with his maid, Teresa (Hira Talfrey). Teresa nurses the girl back to health and tells Alfredo that the girl is with child. However, Teresa worries that the baby will be born on Christmas Day. She claims that for an unwanted child to share his birthday with Jesus Christ “is an insult to heaven.” Sure enough, the baby is born at Christmas and the girl dies shortly afterwards. At Leon’s baptism, the sky darkens and a thunderstorm starts, the font water bubbles and the image of a gargoyle is seen reflected in the water.

Alfredo and Teresa raise the young boy, whom they named Leon (Justine Walters). Then Pepe Valiente (Warren Mitchell), the town watchman, finds a goat with its throat torn out and Teresa finds Leon’s kitten torn to pieces. When Dominique (George Woodbridge) the goatherd reports the killings to Don Enrique (Peter Sallis) the mayor, Pepe is ordered to kill the wolves deemed responsible. That night Pepe shoots at a young wolf. At the Corledo home, Alfredo removes a bullet from Leon’s chest. Teresa is concerned: she did not hear him leave the house. Leon claims to have been in bed all night but had a bad dream. He has had them every night since Pepe took him hunting. Pepe killed a squirrel and Leon had picked up the animal to kiss it. Since he tasted its blood, he wants to taste more. At night he dreams he’s a wolf drinking blood. Alfredo looks at Leon’s palms and sees they are hairy.

Alfredo consults the local Priest (John Gabriel) who tells him that Leon is a werewolf possessed by the spirit of a beast that should not have been born. The spirit and Leon’s soul are at war. Whatever weakens the soul: vice, greed, hatred, solitude, especially at the full moon when the forces of evil are strongest, bring the spirit of the wolf to the fore. Whatever weakens the spirit of the beast: warmth, fellowship, love, raises the human soul. Leon requires love more than ordinary children. When he grows older, if a young woman should love him profoundly, then he may be cured. Alfredo pledges to give Leon the fatherly love he needs. He places bars at Leon’s bedroom window to prevent his escaping again.

An old drunk (Michael Ripper) in the bar claims that the attacks on the livestock were not made by an ordinary wolf but by a werewolf. Pepe melts down his wife Rosa’s (Anne Blake) crucifix to make a silver bullet. That night, he shoots Dominique’s dog dead and the attacks cease.

Leon grows into a young man (Oliver Reed) and leaves home to seek work at the Gomez vineyard. As he arrives, a carriage leaves and splashes his clothes. Don Fernando (Ewen Solon) sets Leon to work in the wine cellar with Jose Amadayo (Martin Matthews) with whom he quickly forms a friendship. The carriage returns and Fernado’s daughter Cristina (Catherine Feller) alights. She comes straight to the wine cellar to apologise to Leon for splashing him. She hopes he will be happy working there. Although Cristina is betrothed to Rico Gomez (David Conville), the son of the vineyard owner, she and Leon fall in love. Every night she has Rico drive her home early complaining of a headache, just so she can spend some time with Leon. He wants her to run away with him but she is reluctant to disobey her father.

On Saturday night, Jose takes Leon to an establishment run by Senora Zumara. They drink, and Jose dances with the girls but Leon is distracted. He begins to feel unwell and goes onto the terrace for some air. The full moon is rising. Vera (Sheila Brennan), a girl from inside, offers Leon her bed to lie down on and takes him to her room. She starts to loosen his clothes and he embraces and kisses her passionately. Then he bites her neck and she complains; but then he changes into the wolf and kills her. Jose goes looking for him and becomes his next victim. Walking the street, the goatherd Dominique becomes Leon’s third killing of the night.

Alfredo finds Leon at home in his bed. The bars at the window are twisted and broken inwards. Leon has no recollection of the night’s events and finds it hard to believe when Alfredo tells him that he is a werewolf. The Priest suggests that Leon be put in chains until he can be admitted to a monastery. Leon flees back to the vineyard, where he is questioned by a police sergeant about the events at Senora Zumara’s. That night, Leon feels the change coming on again. Cristina appears and he screams at her to get away. He trips and knocks himself out. Cristina stays with him all night, and when he awakes no change has occurred. Leon is convinced that she has saved him and begs her to go away with him. She agrees to marry him.

As Leon prepares to leave, the police arrive and arrest him for murder. Locked in a cell, he bribes the gaoler (Denis Shaw) to summon Alfredo. Fernando and Rico catch Cristina preparing to leave. Her father tells her that Leon is in prison where he belongs. Cristina locks the two men in the wine cellar and takes Rico’s carriage to the jail. Leon begs Alfredo to tell the mayor to execute him. Alfredo and the Priest want to take him away and warn the mayor that Leon is a werewolf. Cristina arrives and announces that she is engaged to marry Leon but cannot corroborate the werewolf story. The mayor is unconvinced and refuses to release Leon.

Leon sends Alfredo to get Pepe’s silver bullet, and Cristina is taken to stay with Teresa. As the full moon rises, Leon transforms into the beast. He kills the old drunk and the gaoler and breaks out of the prison. Pursued by a mob, Leon climbs the bell tower, with Alfredo in pursuit. As the villagers ring the bells, driving Leon into a frenzy, Alfredo shoots him dead and covers Leon’s body with his cloak.