In Swinging London, Chris, Frankie Avalon leads a group of twenty-something friends who drift from a dull party to a late-night urge for thrills at an old, supposedly haunted mansion where one of their number used to play as a child. The circle includes Sheila, Jill Haworth — his bored girlfriend; Sylvia, Gina Warwick — who has her eye on the handsome, two-timing Gary, and Dorothy, Carol Dilworth — his “good girl” date. Also along are Madge, Veronica Doran, her nervous, heavy-set boyfriend Peter, Richard O'Sullivan as Peter, the sweet-faced Richard, Julian Barnes and his friend Henry, Robin Stewart. They are followed by Bob Kellett, George Sewell — Sylvia's older, jealous and married ex-boyfriend, who adds a charged undercurrent to the night.
They explore the mansion, even holding a séance before splitting up by candlelight under the moonlit sky. One by one they drift into isolated pockets of the vast, creaking house, the air thick with nerves and the faint crackle of old floorboards. Sylvia, frightened by the mansion, leaves and hitchhikes toward home, but Bob Kellett hangs behind at the mansion, a shadowy reminder of the tensions that simmer beneath the surface.
In the days and weeks that follow, the mood among the survivors grows tenser and more guilt-ridden. When Gary is reported missing, police scrutiny tightens the net and suspicion begins to circle back toward the group. Kellet confronts Sylvia, learning that she may have lost a lighter that could link them to the mansion. He returns there but is killed, leaving the others reeling with fresh fear and unanswered questions.
Dorothy calls everyone together to ask for confessions, but Chris urges them to return to the house to unmask the killer before they all fall prey to a brutal fate. Meanwhile, Sylvia is visited by the police again, and she reveals 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.
Richard recounts how he was locked in a basement for three days as a child and reveals a paralyzing fear of the dark. Despite Chris’ best efforts to keep the group united, he is knifed, and Sheila is frantically chased around the mansion. Just as Richard is about to strike, the moon slides behind a cloud, triggering a flash of childhood fear that saves Sheila in the nick of time, and the arriving police bring the nightmare to a tense, resolve-filled close.
