Novelist William Peter Blatty based his best-seller on the last known Catholic-sanctioned exorcism in the United States. Blatty transformed the little boy in the 1949 incident into a little girl named Regan, played by 14-year-old Linda Blair. Suddenly prone to fits and bizarre behavior, Regan proves quite a handful for her actress-mother, Chris MacNeil (played by Ellen Burstyn, although Blatty reportedly based the character on his next-door neighbor Shirley MacLaine). When Regan gets completely out of hand, Chris calls in young priest Father Karras (Jason Miller), who becomes convinced that the girl is possessed by the Devil and that they must call in an exorcist: namely, Father Merrin (Max von Sydow). His foe proves to be no run-of-the-mill demon, and both the priest and the girl suffer numerous horrors during their struggles.