After several unsuccessful psychiatric and medical treatments, Regan's mother, an atheist, turns to a local Jesuit priest for help as Regan's personality becomes increasingly disturbed. Father Damien Karras, who is currently going through a crisis of faith coupled with the loss of his mother, agrees to see Regan as a psychiatrist, but initially resists the notion that it is an actual demonic possession. After a few meetings with the child, now completely inhabited by a diabolical personality, he turns to the local bishop for permission to perform an exorcism on the child.