Sister Gertrude, a nun working in a Catholic hospital, begins to struggle with her faith and sanity after undergoing brain surgery. Suffering from intense headaches and paranoia, she becomes addicted to morphine and starts exhibiting erratic behavior. Her once compassionate demeanor shifts as she engages in cruel and sadistic acts towards patients and staff. The hospital staff grows increasingly worried about her unpredictable outbursts and disturbing actions. Concurrently, Gertrude begins having macabre hallucinations and violent fantasies, which she attributes to her condition.

As the institution reels from a series of mysterious deaths, the line between Gertrude's sanity and madness blurs further. The hospital is thrown into chaos as suspicions rise, leading to increasing tension amongst the remaining staff. With her behavior becoming more menacing, the staff members are caught in a web of fear and confusion, trying to discern the truth behind the string of fatalities. Eventually, Gertrude's descent into darkness threatens to consume everyone around her, propelling the story towards an unexpected and chilling climax.