Actress Chris MacNeil becomes increasingly concerned when her twelve-year-old daughter, Regan, begins displaying violent and inexplicable changes in her personality and behaviour. After medical treatments fail to identify the cause, Chris turns to Father Damien Karras, a Jesuit priest struggling with his faith and the recent death of his mother. Convinced that Regan may be possessed by a demonic entity, Karras seeks the assistance of the experienced exorcist Father Lankester Merrin.