Famous Hollywood star Chris McNeil is worried about her 12-year-old daughter Regan, who has become the victim of a strange psychosis. The girl begins to speak with someone else’s voice and does terrible things she could not possibly know. Chris believes her daughter is possessed by a demon and turns for help to Father Damien Karras, a Jesuit priest. Although the priest is inclined to psychological explanations, he comes to believe Regan truly is possessed—and decides to call an exorcist.