A movie actress taking up temporary residence in Washington D.C. has her troubles. The script for the movie she's filming seems inadequate. Her ex, who is also the father of her adolescent daughter, Regan, neglects to call the girl on her birthday. And the attic has rats. Meanwhile, Father Karras, a priest and a psychiatrist, is losing his faith; and he's dealing with a sick mother who needs medical care he hasn't the money to provide. Another priest, the old and ailing Father Merrin, has just returned from Iraq with forebodings of evil. These three persons meet when the sweet and cheerful Regan turns foul-mouthed and violent. But her sickness is beyond the reach of a medical doctor or a psychiatrist. What Regan needs is an exorcist.