Bloodbath at the House of Death

The film opens in 1975 at a place called Headstone Manor, which is being used as a "businessman's weekend retreat and girls' summer camp". A few minutes into the film, a group of satanic monks enter the house and kill 18 of its occupants.

In 1983, Doctor Lucas Mandeville (Kenny Everett) and Doctor Barbara Coyle (Pamela Stephenson) are sent to investigate radioactive readings in the area that have been traced to Headstone Manor, now known by locals as the House of Death.

Along with several other scientists, Mandeville and Coyle set up their equipment in the house, while the Sinister Man (Vincent Price), a 700-year-old Satanic priest, prepares a rite in the nearby woods to purge the house of its unwanted guests.

During this time, Mandeville reveals that he was once a successful German surgeon named Ludwig Manheim, who was reduced to "smart-arse paranormal research crap" after a humiliation in the past. Coyle also encounters a poltergeist, and the two engage in sexual intercourse.

Several satanic clones of Mandeville, Coyle and the other scientists enter the house, and begin killing off the originals and taking their place. When Coyle is about to be killed, she is rescued by the poltergeist and saved. The satanic monks then take off in a spaceship, revealing that these monks are aliens using the house for their activities on Earth. The film ends with the spaceship soaring into the skies, with an E.T. voice groaning: "Oh, shit! Not again!".