Six weeks after the death of her mother, Nancy Perkins [Sandra Harrison]'s father [Thomas Browne Henry] marries Doris [Jean Dean] and enrolls the 18-year-old smoker Nancy into the Sherwood School for Girls, a private boarding school with an outstanding reputation. They are greeted by the principal, Mrs. Thorndyke [Mary Adams], who stresses that the school is not a corrective institution but a prestigious preparatory academy where discipline, tradition, and high standards are valued above all.
Nancy’s arrival unsettles the dormitory, and she is subjected to cruel hazings by the other girls that first night. At breakfast the next morning, Myra [Gail Ganley], the dormitory’s leader, reassures Nancy and hints at a tighter, more secret social structure. She mentions their clandestine club, The Birds of Paradise, and introduces Eddie [Don Devlin], a young groundsman whom the girls swap attention with in rotation. Myra also works as an assistant to Miss Branding [Louise Lewis], the school’s chemistry teacher who is quietly drafting a thesis about a “terrible power”—“strong enough to destroy the world — buried within each of us.” Branding hopes to persuade the scientific community to turn away from weapons research, and she signals that she is searching for a special girl to test her theory on. Myra suggests Nancy as the ideal candidate.
In chemistry class, Myra and Nola [Heather Ames] deliberately swap a chemical, causing Nancy to react violently. Later, Branding speaks with Nancy and gradually earns her trust. She places an ancient amulet around Nancy’s neck, claiming it hails from the Carpathian Mountains and that it can both heal and destroy, capable of unlocking frightening powers. Under Branding’s instructions, Nancy learns to obey, and the hypnotic grip of the amulet grows stronger.
During an initiation party, Eddie and two local boys, Tab [Jerry Blaine] and Joe [Jimmy Hayes], creep into the girls’ room. Branding, already sensing trouble from afar, re-hypnotizes Nancy, turning her into a vampire and setting off a chain of deadly events. The party is interrupted by Miss Rivers [Edna Holland], who sends Nola to the basement to fetch supplies. In the basement, Nola is attacked by something subhuman and is killed.
The next morning, Detective-Sergeant Stewart [Richard Devon] begins to investigate the murders. In the corner, the coroner, Dr. Lawson [Voltaire Perkins], reports two puncture wounds in Nola’s jugular vein and notes that the body was drained of blood. Mike [Paul Maxwell], a young assistant to the coroner who once shared a med-school room with “an exchange student from a small town in the Carpathian Mountains,” recalls similar vampire tales from that friend, though Lt. Dunlap [Malcolm Atterbury] remains skeptical.
As Halloween approaches, the girls organize a cemetery scavenger hunt. Nancy is transformed again into a vampire and kills Terri [Shirley Delancey] and Tab. The police round up the participants for lie-detector tests, but Branding manipulates Nancy’s answers through remote hypnosis, keeping the murderous truth in shadow.
Back at the school, Nancy is frightened by her own transformations and pleads with Branding to end the experiment. Branding assures her that the project will soon be over and that she will be proud of her role in preventing humanity from self-destruction. The state pressures the school to close amid the murders, and Mrs. Thorndyke decides Branding should assume more duties to calm anxious parents while she negotiates with authorities. Nancy’s boyfriend from back home, Glenn [Michael Hall], arrives to check on her, only to be met with a chilling coldness that stems from her fear of an urge to kill someone she cares about.
After leaving Glenn, Nancy returns to Branding’s laboratory, begging for release. Branding, consumed by her own obsession, refuses and hypnotizes Nancy once more. Nancy becomes a brutal vampire again and attacks Branding, strangling her with the amulet’s chain. In the struggle, Branding pushes Nancy away; the force causes Nancy to fall and be impaled on a broken piece of furniture. At that decisive moment, Glenn, Mrs. Thorndyke, and Myra break through a locked door to witness Branding’s ruined thesis, its pages soaked in acid and shredded, a grim symbol of the project’s failure and the danger it unleashed.
