The terror begins unobtrusively. Noises in the attic. In the child's room. An odd smell. The displacement of furniture. An icy chill. Easy explanations are offered. Then frightening changes are seen in eleven year old Reagan. Medical tests shed no light on her symptoms, but it is as if a different personality has invaded her body.
A Jesuit priest, Father Damien Karras, is called in. Is it truly possible that a demonic presence has possessed the child? Exorcism seems to be the only answer...