Leslie Shaber, Susan Blakely, is an average suburban housewife who often feels overlooked and taken for granted by her husband Howard John Schuck. On a casual outing to the pet store, Leslie meets the store’s enigmatic and handsome owner, Harry Thropen, John Saxon, whose attention flusters her in a way she hasn’t experienced in years. After a purse-snatching scare that Harry intervenes to prevent, he invites Leslie to lunch, and their conversation quickly grows more intimate. During the meal, Harry plants a bold kiss on Leslie that stuns her, and the moment is witnessed by her daughter Jennifer Tina Caspary, who reads the scene as evidence of an affair. The kiss ends abruptly when Harry is startled by their flambé dessert, but the impression sticks.
Leslie’s world tilts when she follows Harry back to his store, where he exerts a strange, almost hypnotic influence. He leads her to a private space that feels both inviting and dangerous, and there he bites her toe in a way that jolts her out of the trance. The unsettling moment leaves her shaken, and as she staggers away, Harry coolly states that he will “be in her thoughts.”
Back home, Leslie undergoes a dramatic change. Her demeanor shifts, and she abandons her vegetarian habits for meat, while her appetite turns predatory and her sexual life grows more intense. That night, she is haunted by vivid nightmares about Harry, and upon waking, she discovers startling physical changes: fangs sprouting in her mouth, and a creeping fur that blankets her skin. Her ears elongate into pointed shapes, and she struggles to remove the hair that now covers much of her body. Jennifer, who has been watching closely, realizes that her mother is transforming into something supernatural.
Before Leslie can learn more, Harry appears again and uses his powers to ease Jennifer into leaving him alone with her mother. Leslie manages to resist his seductions for a time, but the tension crescendos when Howard returns home and Harry exits, warning that her husband will not accept her in her current state. Terrified by the unfolding transformation, Jennifer seeks help from her horror-movie-loving friend Stacey Diana Barrows, who remains skeptical, and from a gypsy who had previously warned that strange things would happen to someone close to them. The gypsy’s guidance proves unreliable, adding to the confusion.
The next day, Leslie appears normal again and resumes her routine, with Jennifer and Stacey secretly tailing her. A trip to the beauty salon, however, triggers another transformation, convincing Stacey that the nightmare is real. The two teens decide they must act before the moon sets or the curse will become permanent. They confront Harry, hoping to end his hold over Leslie, but they are met with dangerous myth and misdirection from the gypsy’s advice.
Against the odds, Jennifer manages to kill Harry, freeing her mother from the curse. The ordeal leaves Howard feeling guilty and determined to be a better partner, while Stacey warns Jennifer that they must perform an exorcism on Harry’s body to prevent a future relapse. Exhausted and tense, Jennifer finally yawns, revealing long fangs that hint at the danger that still lurks within her family.
