Actress Chris MacNeil, concerned about her daughter's strange behavior, seeks medical help. Test results confirm the initial diagnosis that Regan is completely healthy. Meanwhile, the girl's condition worsens. When the actress returns home from a filming session, her daughter suddenly attacks her mother, who is forced to tie her to the bed. Desperate, Chris now seeks help from a young priest, Father Karras. He concludes that Satan has possessed Regan. Wanting to exorcise the devil, Karras calls upon the experienced exorcist, Father Merrin.