Actress Chris MacNeil becomes increasingly disturbed by the inexplicable behavior of her 12-year-old daughter Regan--mouthing obscenities, predicting the murder of one of Chris's friends, and urinating in public. Doctors and psychologists have no explanation and as a last option, Chris consults a priest. Father Damian Karras, a Jesuit who is having doubts about his own faith following his mother's death, investigates. Karras comes to no other conclusion...