Accompanied by her 12-year-old daughter Regan, actress Chris MacNeil relocates to Washington D.C. where she is filming a movie. Mother and daughter have a good relationship, but after a time Regan begins to act strangely. She undergoes various neurological tests, but doctors can find nothing to explain her behavior. As Regan's situation reaches crisis proportions, Chris turns to Father Karras, a Roman Catholic priest and psychiatrist, to see if an exorcism might solve their problem. Karras is incredulous, but the church eventually agrees calling in Father Merrin, who had previously conducted an exorcism and came face to face with the demon.—garykmcd