THE HAUNTED HOUSE OF HORROR

In Swinging London, a group of twenty-something friends are attending a rather dull party, and they decide to gather for kicks at an old, supposedly haunted mansion where one of their number used to play as a child. Among the group is American ringleader Chris, his bored girlfriend Sheila, promiscuous Sylvia (who has her eye on handsome two-timing Gary) and his "good girl" date, Dorothy. Also tagging along are nervous, heavy-set Madge, her sarcastic, hot-tempered boyfriend Peter, sweet-faced Richard and his friend Henry. They are all followed by Paul Kellet, Sylvia's older, jealous and married ex-boyfriend.

They have fun exploring the mansion, even holding a séance before separating one by one by candlelight on the moonlit night. Sylvia, frightened by the mansion, leaves and hitchhikes toward home, but Kellet hangs behind at the mansion. While all the partiers are alone, Gary is brutally knifed and his body is discovered by the panic-stricken Dorothy and the others. Because some of them have a criminal record, Chris convinces the group to leave Gary's body far from the home and to pretend that Gary left and that no one knows where he went. They are all shaken by Chris' assertion that one of them must be the murderer.

During the next few weeks, the survivors are possessed by tension and guilt, and after Gary is reported missing, they are further shaken by questioning from the police. Kellet confronts Sylvia, learning that she may have lost a lighter that could link them to the mansion. He returns there but is also killed.

Dorothy calls the survivors together to ask to confess. However, Chris convinces them to return to the house to discover who among them is the killer before they all succumb to a gruesome death. Meanwhile, Sylvia is visited by the police again, and she discloses the location of the house after learning of Kellet's disappearance. At the mansion, Dorothy becomes hysterical, prompting several of the group to depart, leaving just Chris, Sheila and Richard. While Sheila is out of the room, Richard recounts how he was locked in a basement for three days as a child and tells that he has a paralyzing fear of the dark. Despite Chris' efforts, he is also knifed and Sheila is frantically chased around the mansion. Just as Richard is about to strike, the moon goes behind a cloud, bringing about his reversion to childhood and fear of the dark, thus saving Sheila as the police arrive.

Richard is revealed as the killer, but his paralyzing childhood fear of the dark resurfaces, saving Sheila as the police arrive.

The climax hinges on Richard's traumatic past: his fear of darkness overtakes him as he attempts one last attack, leaving Sheila unharmed. The ending ties together the film’s themes of repressed trauma and hidden dangers within familiar circles, while providing a resolution that both explains the motive and spares the last intended victim. The arrival of the police closes the murderous chapter, reinforcing the dangers of secrets and unchecked psychological wounds.