Sister Gertrude, a nun who works in a Catholic hospital for the elderly, returns to work after healing from surgery to remove a brain tumor. Gertrude suffers from significant anxiety following her surgery, and believes that her cancer has returned, despite the hospital's lead doctor, Dr. Poirret, assuring her there is no evidence for it. The Mother Superior of the convent also dismisses Gertrude's fears as hypochondria. Sister Mathieu, who has an unspoken sexual attraction to Gertrude, believes Gertrude's worry is legitimate.
Unbeknownst to the convent, Gertrude begins leading a double life, venturing into the city and pursuing sexual encounters with random men. Gertrude also begins to intravenously abuse morphine and heroin that Sister Mathieu steals for her from the hospital. A change in her personality is noted by Dr. Poirret, as Gertrude begins to mistreat patients and revel in reading gory hagiography on the lives of tortured saints to them. Gertrude successfully manages to have Poirret fired from the hospital. While Gertrude is in a drug-induced state, Sister Mathieu's grandfather, a patient at the hospital, is bludgeoned to death with a lamp, and his body thrown out a window so as to appear as a suicide. Sister Mathieu finds Gertrude's veil with his body, but, pledging her loyalty, assures Gertrude she will not implicate her, promising to burn it to hide the evidence.
Following Dr. Poirret's dismissal, the young and handsome Dr. Patrick Rowland is hired as his replacement. During a rainstorm, Gertrude witnesses Jonathan, an elderly male patient, having sex with a young female orderly outside. Later, she awakens from a nightmare in which she suffocates Jonathan, though she is unable to discern whether it was merely a dream or if she might have actually committed another murder. Jonathan's corpse is found the following morning lying in the grass. After leading a prayer for Jonathan, Gertrude has a nervous breakdown. Shortly after, Janet, another patient, is bound and gagged by an unseen assailant before being ritualistically stabbed in the face with needles and slashed in the head and neck by a scalpel.
After Janet's body is found hanging in an elevator shaft, Dr. Poirret sedates a panicked Gertrude. When Gertrude awakens, she confronts Peter, a middle-aged patient on crutches, who claims to have knowledge of who is committing the killings. Gertrude believes him, assuming she is being framed, but he will not reveal any further details to her. In response, she steals his crutches from him. After Peter's body is found in the boiler room, Gertrude is escorted out of the hospital and met by the Mother Superior, who scolds her for her behavior and has her sent to an isolation cell to be sedated. Meanwhile, Sister Mathieu confesses to Dr. Roland to stealing drugs for Gertrude, and threatens suicide if she is exposed. She then begins to seduce him.
Meanwhile, isolated in her cell, Sister Gertrude sits in a catatonic state, detoxing from her drug abuse. As she regains mental clarity, she recalls the first murder committed in the hospital, of Sister Mathieu's grandfather. Gertrude realizes that it was in fact perpetrated by Sister Mathieu, as Gertrude watched in an intoxicated state. Gertrude, psychologically fragile and in a state of perpetual drug use, mistakenly assumed she was committing each of the murders, when it was in fact Sister Mathieu, motivated to kill by the sexual abuse she suffered as a child by her grandfather.
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