Something beyond evil is happening in a little girl's room. Regan (Blair) has brutally changed both in the way she looks and the way she acts, with violent outbursts on everyone who comes in contact with her. Her worried mother (Burstyn) gets in contact with a priest (Miller) who comes to the conclusion that Regan is possessed. The top priest who can deal with an exorcism, Father Merrin (Sydow) is called in to help save Regan from the demon inside her.