Regan (Linda Blair), the adolescent daughter of a movie actress (Ellen Burstyn) residing in Washington D.C., is apparently possessed by an ancient demon. When all medical avenues are exhausted, her mother goes to a priest who is also a psychiatrist.

He realizes Regan is possessed and he and another priest experienced in exorcism risk their lives to administer the rites of demonic exorcism and drive the spirit away from Regan before she dies.