Its plot follows a nun who, after recovering from brain surgery, grows increasingly paranoid that her health is again declining; she begins indulging in opioids from the hospital in which she works, and spirals into addiction and madness with violent consequences.

The film is loosely based on the true story of Cécile Bombeek, a middle-aged nun who committed a series of murders in a geriatric hospital in Wetteren, Belgium in 1977.

Dr. Poirret notes a change in her personality as Gertrude begins to mistreat patients and revel in reading gory hagiography on the lives of tortured saints.

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 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.

Gertrude, psychologically fragile and in a state of perpetual drug use, mistakenly assumed she was committing each of the murders when it was Sister Mathieu motivated to kill by the sexual abuse she suffered as a child by her grandfather.

The film is loosely based on Cécile Bombeek,[2] a middle-aged nun who became addicted to morphine and committed a series of murders in a geriatric hospital in Wetteren, Belgium from 1976-1978.

In the United Kingdom, Mary Whitehouse denounced it as one of the "video nasties" subgenre of violent horror cinema, which "might" adversely affect human behaviour.

Although it was originally on a "DPP list" of "objectionable" films in the United Kingdom, compiled by the Director of Public Prosecutions in 1983 as a result of the aforementioned moral panic and released with 13 seconds of cuts in 1993.