Traumatized by her mother's death, young Susan is becoming possessed by the same demon that possessed her mother before she died. More and more her husband and psychiatrist are noticing the strange changes...
- Young Susan Walker (Julie Christie Murray) attends her mother's funeral with her aunt, Cora Nomed (Laura Hippe). However, Susan refuses to leave with Cora and runs back into the cemetery, where she discovers the Nomed mausoleum. As the gate springs open, she enters, finding herself surrounded by green fog and the sound of thunder. When a tramp wanders in behind her, Susan's eyes turn luminous green and a mysterious man in a robe hits the tramp with some kind of spell. The tramp staggers from the structure with smoke and flames emanating from his face, resulting his head exploding. Susan kneels before the tomb and touches it, causing the lid to open and a claw-like hand to reach out.
Twenty years later, psychiatrist Dr. Simon Andrews (Norman Burton) assures Cora that her ongoing concerns about Susan (Bobbie Bresee) are unfounded. Cora counters by handing Simon her father's history of the Nomed family, which describes a 300-year-old family curse. Simon dismisses the story as superstition, ignoring Cora's insistence that she has witnessed similarly erratic behavior in both her late sister and her niece. Despite Simon's skepticism, he reads the book, which explains how a demon "broke free of the crown of thorns" and now possesses the body of the firstborn daughter to each generation of Nomeds. It also states that the demon can only be cast out by reuniting it with the crown of thorns, which adorns the gate of the mausoleum.
That evening, Susan's husband, Oliver Farrell (Marjoe Gortner), awakens her from a nap and offers to take her dancing to a nightclub, as their gardener, Ben (Maurice Sherbanee), watches with interest through a window. Shortly after arriving at the nightclub, Oliver is called back to his office, and the couple is followed to the exit by a drunken man, who is enamored by Susan. Offended by the man's rudeness, Susan follows him with her glowing green eyes and causes his car to burst into flames once he is inside. Oliver tries to rescue the man, but is unable to open the car doors. The drunken man burns to death inside his car.
Ben appears at the Farrell home the next day and flirts openly with Susan while her husband is at work. She responds in kind, and their ensuing sexual encounter concludes with Susan using a garden claw to kill and mutilate Ben. The following morning, Cora arrives at the house, asking for Susan's signature on some estate papers. The demon overtakes Susan and uses powers of telekinesis to levitate Cora off the the floor and makes her torso split open, killing her as well.
Oliver is unaware of his wife's alter ego until later that night, when he awakens to find Susan asleep in a rocking chair, bearing the demon's face. He telephones Simon, who maintains his skepticism and offers an appointment for the next day. Susan returns to normal, but Oliver is unable to sleep.
In the morning, he asks Elsie (LaWanda Page), the maid, to look after Susan until he returns in the early afternoon. However, the green fog and pulsating light that surround Susan only encourage the maid to pack her suitcase and leave, never to return.
Although Susan tells her husband that nothing is wrong, she keeps her appointment with Simon. The psychiatrist places her under hypnosis and records the session, during which the demon reveals itself. Afterward, Simon plays the tape for his colleague, Dr. Roni Logan (Sheri Mann), a specialist in the paranormal, who convinces him of Susan's demonic possession.
Meanwhile, Susan invites a delivery boy (Chu Chu Malave) into her home with the promise of sex, but she transforms into the demon and uses powers to telekinesis to split his skull, killing him too.
The next day, Simon informs Oliver of Susan's condition and instructs him to avoid close contact with her until the crown of thorns has been retrieved. That afternoon, Susan goes to a local shopping mall where she steals a painting from an art gallery. When the salesman pursues her, she uses her demon powers to levitate him high above the floor, then allows him to drop onto a conical metal sculpture on the ground level.
Upon his return home, Oliver expresses dislike for the painting, which features a centaur and bare-breasted women. The tension between the couple continues through dinner, but Oliver later apologizes to Susan while she is taking a bubble bath, and she climbs from the tub to embrace him. A thunderstorm rages outside as Simon removes the crown of thorns from the mausoleum gate, and Susan tightens her embrace until it crushes Oliver's body (her bare breasts turn into two individual demon heads which their mouths filled with sharp teeth disembowel Oliver).
Simon arrives at the house a little later to find Oliver dead in the bathtub and Susan playing with dolls in the attic. As Susan reminisces about her childhood, Simon places the crown of thorns on her head, removing the demon from her body and returning it to the cemetery. Simon and Susan arrive at the mausoleum, where they find the creature sprawled over the tomb. At the urging of Simon, Susan places the crown on the demon's head, and the tomb opens, trapping the monster inside as the sun rises outside.
Susan emerges from the experience in a state of bewilderment, and Simon promises to explain everything over time (refusing to tell her about Oliver's fate). Before leaving the cemetery, Simon tells the caretaker (the same robed being from earlier), "You've known this secret and lived with it, like your father before you and his father before him," reminding him that no one is to enter the mausoleum. As Simon and Susan drive away, the caretaker reveals himself to be Ben, the gardener. He looks at the camera, breaking the fourth wall, and laughs.