The Haunted House of Horror, also titled Horror House and The Dark is a 1969 British film, an early type of teen "slasher film". It starred Frankie Avalon and Jill Haworth as young adults looking for a thrill by spending the night in an old mansion in the English countryside. Avalon was nearly 30 years old at the time of filming. It was directed by Michael Armstrong who would go on to direct Mark of the Devil.
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 partygirl 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 and her sarcastic, hot-tempered boyfriend Peter, sweet faced Richard, and his friend Henry. They are all followed by Paul Kellet, Sylvia's older jealous married ex-boyfriend.
They have fun exploring the mansion, even holding a seance 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; his body is discovered by the panic-stricken Dorothy and the others. Since 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 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 at the mansion. He returns there where he 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 a tale of how he was locked in a basement for three days as a child and how he has a paralyzing fear of the dark or anyone he suspects will lock him away. 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.
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The film's box office performance was reasonable.