Sister Gertrude, Anita Ekberg, a nun who works in a Catholic hospital for the elderly, returns to work after brain tumor surgery, carrying a heavy cloud of anxiety. She is convinced that her cancer has returned, even as the hospital’s lead doctor, Dr. Poirret [Massimo Serato], assures her there is no medical evidence of recurrence. The convent’s Mother Superior, Alida Valli, dismisses her fears as hypochondria, while Sister Mathieu, who has an unspoken attraction to Gertrude, believes Gertrude’s worry is legitimate.
Unbeknownst to the others, Gertrude begins living a double life. She slips into the city to seek sexual encounters with random men and, in a troubling turn, begins to intravenously abuse morphine and heroin that Sister Mathieu steals for her from the hospital. Dr. Poirret notes a troubling change in her personality as she starts mistreating patients and immerses herself in reading graphic hagiographies about the lives of tortured saints. The strain of her behavior eventually leads to Poirret being fired from the hospital.
In a drug-fueled episode, Sister Mathieu’s grandfather, a patient at the hospital, is bludgeoned to death with a lamp and his body is defenestrated to look like a suicide. Sister Mathieu discovers Gertrude’s veil with the body and, swearing loyalty, vows not to implicate her, promising to burn the evidence to hide what happened.
After Poirret’s dismissal, the hospital hires a new physician, Dr. Patrick Roland, Joe Dallesandro. In a stormy night scene, Gertrude witnesses Jonathan, an elderly patient, having sex with a young female orderly outside. She later awakens from a nightmare in which she suffocates Jonathan, but she cannot tell if it was merely a dream or a murder she has committed. The next morning, Jonathan’s body is found in the grass, and Gertrude leads a prayer for him before suffering a nervous breakdown. Shortly after, Janet, another patient, is bound and gagged by an unseen assailant and later stabbed in the face with needles and slashed in the head and neck with a scalpel.
After Janet’s body is discovered hanging in an elevator shaft, Dr. Roland sedates a panicked Gertrude. When she awakens, she confronts Peter, a middle-aged patient on crutches, who claims to know who is committing the killings but will not reveal further details. Gertrude believes him and even steals his crutches, but Peter’s body is later found in the boiler room. Gertrude is escorted out of the hospital and confronted by the Mother Superior, who scolds her and has her sent to an isolation cell to be sedated.
Meanwhile, Sister Mathieu confesses to Dr. Roland about stealing drugs to help Gertrude and threatens suicide if she is exposed. She then begins to seduce him, intensifying the hospital’s tense, morally ambiguous atmosphere. Isolated in her cell, Gertrude detoxes from the drugs and, as clarity returns, recalls the first murder—the grandfather killed by Sister Mathieu. Gertrude realizes, with mounting horror, that she had been intoxicated and misattributing the killings to herself, while Sister Mathieu was the true killer, driven by the sexual abuse she suffered as a child at the hands of her grandfather.
As the truth surfaces, the film reveals a devastating dynamic: the apparent string of murders at the hospital is rooted in a cycle of trauma, manipulation, and self-deception. Gertrude’s drug-soaked perception leads her to believe she is the killer, only to uncover that Sister Mathieu was acting from a history of abuse and a desperate response to her pain. The resolution leaves the audience with a haunting meditation on guilt, memory, and the blurred line between victim and predator within a claustrophobic, institutional setting.
