Dr. Gene Tuskin works with troubled children and the most troubled among them is Regan MacNeil. She is obsessed with the thought of a demon inside of her. The doctor links himself with the girl by a special divice and finds out the demon is real: he is Pazuzu. Regan's exorcist, father Merrin, died while performing an exorcism for her. When Father Philip Lamont is assigned to investigate the death, he follows Merrin's step to Africa and finds out a deadly secret.