As cries of terror ring out in the night at the LaSalle Detention Home for Girls, Eric, a ghoul-like member of the staff, scoops up one of the inmates and takes her to Dr. Murdock, the head of the institute, who straps her onto a table. The next day, Mrs. Ford, the institute’s stern administrator, diagnoses Anna Sherman, one of the new girls, with a heart condition and sends her to the infirmary. After Big Marge, a wise-cracking inmate, questions her judgment, Mrs. Ford sentences her to isolation. When word comes that one of the girls died of a heart attack the previous evening, Tracy, another one of the prisoners, wonders about the home’s extraordinary death rate. Carol Adams, the young staff social worker, notes Tracy’s concerns and decides to conduct her own investigation. As Carol studies the school’s files, Mrs. Ford enters and demands an explanation. After Mrs. Ford reports Carol’s actions, Murdock sends Eric to fetch Anna. In a hidden attic lab, they lower the screaming Anna in a tub of water, hook her up to some equipment and then drain the life from her to transfuse Eric. When the girls return from watching a movie that night, they find Anna’s lifeless body dangling from the dormitory rafters, an apparent victim of suicide. After Carol questions the coroner’s finding of suicide, Murdock discredits her by suggesting that she is more interested in social activities than the inmates’ mental health. When Murdock implies that Carol is unfit for her position, Dr. Jess Rogers, a young prison board psychiatrist, is assigned to look into the charges, but once Carol tells Rogers about the epidemic of heart attacks plaguing the school, Rogers asks her to join his investigation. As Rogers interviews the inmates, he learns that eleven girls have died in the two years since Murdock and his colleagues--Mrs. Ford, Eric, and Drs. Myer, Cooper and Freneau--took over the institute. When Rogers asks to see the death records, Murdock claims that they were all destroyed in a fire. Suspicious, Rogers requests an autopsy of Anna’s body, and after Murdock refuses, Rogers sneaks into the morgue and performs the autopsy himself. When Murdock interrupts him in the middle of the procedure, Rogers lies that his findings suggest suicide. Afterward, Rogers tells Carol that Anna was already dead at the time of the hanging. Sensing animosity between Cooper and the other staff members, Rogers visits the doctor, who cryptically states that he expects to die suddenly, after which Rogers will find instructions in the mail. Soon after, Murdock convenes a meeting of the staff and announces that they have decided not to renew Cooper’s life because he is a menace to the project. As the dying Cooper gasps for breath, he declares that he has sent his story to someone, but refuses to divulge the name of the recipient. After Cooper keels over dead, Murdock surmises that either Rogers or Carol must be the addressee. Soon after, Rogers finds a letter from Cooper, directing him to a diary hidden in the cliffs. In a locked box, Rogers discovers Cooper’s incredible account of meeting Murdock and his colleagues 170 years earlier when they began experiments in the prolongation of life by transferring electrical energy from one recipient to another. They found that the ideal donor was a nubile young woman who would die once her life force was extracted and the transfer completed. The diary continues that Eric’s experiment went awry, and hence he has exhausted most of his life energy, partially turning to stone. Eric, who has followed Rogers, suddenly appears and tries to pry the diary from his hands. Rogers escapes, however, and Eric then returns into the institute and bursts into the dormitory. As the girls scatter in panic, Eric proceeds to the isolation ward, kills the matron and snatches Marge. At the lab, Eric threatens Murdock’s life unless he performs a transfusion using Marge as a donor. Rogers then sneaks back into the institute and relates Cooper’s story to Carol. With the rest of the staff occupied with trying to round up the hysterical girls, Rogers slips into the now deserted lab. When Murdock appears, Rogers hides and then runs back to alert the police. After Rogers discovers that Murdock has ordered the switchboard closed, he asks Tracy, who has been trained to operate the switchboard, to place a call. As Tracy begins to dial the police, Myer fires a pistol at the switchboard, disabling it. After wrestling the weapon from Myer, Rogers hands it to Tracy and tells her to hold the doctor at gunpoint. Rogers, meanwhile, sneaks into the cellar to disconnect the water and fuses, plunging the institute into blackness. When Eric suddenly arrives carrying Carol as his next victim, Rogers distracts him by setting Cooper’s papers on fire. After momentarily dropping his guard, Eric slugs Rogers and then slings Carol over his shoulder and continues to the lab, where he drops her into the transfusion tank. Regaining consciousness, Rogers crawls into the lab and pours a neutralizing formula into the tank, In a crazed, weakened state, Eric stalks Freneau, who shoots and kills him. Murdock straps Rogers into the chair and forces him to return the fuses, then sends Freneau to the cellar to restore the electricity. When Tracy sees the lights switch back on, a prearranged signal of danger, she and the girls storm the house. Cooper’s papers have ignited into a raging fire in the cellar, and as the girls reach the house, Rogers wrestles Murdock and escapes with Carol. As the house explodes in flames, Murdock and the others remain in their lab to perish in the inferno.