Maggie Walsh and her boyfriend, Pete Danner, architects from Los Angeles, California, are offered a job in England by a mysterious client who advances a generous payment of $50,000. The couple arrives abroad early for the assignment, to spend time sightseeing. While motorcycling in the English countryside, they swerve off the road to avoid a chauffeure-driven Rolls-Royce. Although Maggie and Pete are unharmed, the motorcycle is wrecked. The owner and passenger of the automobile, a charming English gentleman, Jason Mountolive, invites the couple to relax at his home while the motorcycle is repaired in the village. Mountolive’s expansive estate, known as Ravenhurst, impresses Maggie and Pete. As the couple proceeds inside the manor, Mountolive, who appears to be in discomfort and swallows a pill, says he will join them later. Maggie and Pete wander through the residence and are greeted by a nurse named Adams. They are curious why Adams has prepared a bedroom for them, then are told that transportation from the small village will have to wait until tomorrow. Meanwhile, the couple is unaware that Mountolive has suddenly become an invalid and must be assisted upstairs by Harry, the chauffeur. As they unwind, Maggie has a strange feeling that their arrival was somehow expected. After awhile, they hear a helicopter land outside and watch as a group of stylish passengers disembark. Unusual circumstances continue when Pete is scalded in the shower by water that turns dangerously hot. Meanwhile, Adams informs the servants that Mountolive is dying, and one of the maids wonders who will be the new master of the estate. That evening, Pete and Maggie are invited for cocktails and are introduced to the other arrivals, who appear to be regular guests at Ravenhurst: Jacques Grandier, a hotelier, Karl, a German weapons manufacturer, Barbara, a publishing magnate, and Clive, a successful music producer. Pete and Maggie also see an attractive woman swimming in the indoor pool and are told that she is Maria Gabrielli, a prominent Italian hostess. Each guest has realized their career ambitions and is wealthy, thanks to Montolive’s support. Jacques explains that, in exchange for such generosity, the five of them operate Mountolive’s businesses, and he adds that Maggie is now part of their elite group. Meanwhile, Maria dives in the pool, but the surface of the water becomes strangely sealed, trapping her underneath, and she drowns. When Adams informs the guests about the accident, everyone is baffled, since Maria was a champion swimmer, and there was no indication that she hit her head on the bottom of the pool. Maggie is summoned to meet with Mountolive during a ceremony attended by the other guests, who all wear the same ring. In his room, which has been transformed into a state-of-the-art medical facility, Maggie is told to approach Mountolive’s bedside. The sick man reaches out with his hand, which is now contorted and ancient, and places a ring on her finger. Maggie faints at the sight of him. That night, Maggie becomes frantic when she is unable to remove the ring. Pete tries to calm her, then telephones for transportation, but all lines are busy. The following morning, Adams informs Pete and Maggie that they cannot leave while the police are investigating Maria’s death. After Pete arranges for Harry to drive them to the village, the chauffeur inexplicably leaves without them. To escape from the estate, Pete and Maggie steal horses and ride to the deserted village. At the mechanic’s garage, they find the motorcycle still in disrepair. Nearby, they spot the Rolls-Royce unattended and confiscate the car, but the road always leads them back to Ravenhurst. When the car stalls, Maggie and Pete have no choice but to return to the house. Irritated by the strange circumstances, Maggie wonders if they are under the influence of black magic. When Barbara offers a formal gown for dinner, Maggie asks her about the ring. Barbara says the ring symbolizes their debt to Mountolive, who can be trusted to fulfill all her dreams. Maggie realizes that Mountolive is the mysterious client who hired her. At dinner that evening, Maggie makes an impression in the regal gown, and boldly asks Jacques if he and the others are involved in black magic or the occult. Suddenly, Clive chokes on his food, and dies when Adams performs a tracheotomy to remove a bone. As she leaves the disturbing scene and escapes to the library, Maggie notices a sixteenth century portrait of a woman, who bears a striking resemblance to her. Karl enters and explains that she is Margaret Walsingham, who was born at Ravenshurst, and was later taken from the house as a young woman and burned at the stake. Her illegitimate son, Jason of Mountolive, inherited Margaret’s fortune and power. Karl gives Maggie a book written by Margaret and suggests she read it. After Maggie leaves the room, Karl is engulfed in flames from the fireplace and burns to death. Later, Pete finds the German’s charred remains in the courtyard. Meanwhile, Maggie learns from the book that Margaret made a bargain with Satan and one of the ring bearers will be chosen to carry on Margaret’s soul. However, Pete is angry that Maggie appears to believe these tales. In the library, she discovers newspaper clippings about the guests, revealing their criminal pasts. Furthermore, Maria, Karl, and Clive died in a manner that reflected their crimes, and Maggie concludes that Mountolive bought their lives. Since there are no clippings about Jacques, Pete assumes that the hotelier is the murderer. In the meantime, they knock on Barbara’s door to warn her, and the three of them agree to leave together. However, while Barbara is getting ready, she is stabbed to death by shards of glass from a mirror. When Maggie and Pete find her body, Maggie realizes she was the last one to see each victim alive. The next morning, Pete prepares to take the Rolls-Royce to seek help when Jacques shoots at him from the roof. Using a crossbow, Pete hits Jacques with an arrow, and the Frenchman is able to return fire and wound Pete. As Maggie stares at Jacques, his rifle backfires, and he plunges to his death. Maggie is now convinced that she serves as a catalyst for the murders. She enters Montolive’s room where he confesses to instigating the five killings, or sacrifices, so that the sixth guest, Maggie, will inherit his power, wealth, and everlasting life. As part of the bargain with Satan, Maggie must find six souls and kill five when she is ready to die, as the survivor will continue the family legacy. Meanwhile, Pete barges through the room. Nurse Adams attempts to restrain him, but she is thrown down the stairs and crushes the house cat. With a sense of her new authority, Maggie leaves Mountolive’s bedside and assures Pete that everything is fine, then revives the dead cat. Maggie greets the servants, and they acknowledge her as the new master. Outside, she gives Pete one of the rings to wear, and they kiss. He then asks Maggie how she plans to use her newfound powers.