The film is based on events appearing in the 1993 book Possessed by Thomas B. Allen,[1][2][3] which was inspired by the exorcism case of Roland Doe.

William S. Bowdern is a World War II veteran who was severely affected by a bad experience in France on All Saints' Day in 1944.

Years later, Bowdern is teaching his students at St. Louis University (SLU) when, at the end of the lecture, a frozen sausage smashes through the classroom's window.

Father Raymond McBride pays his bail, and drives him to the Alexian Brothers Hospital to show where the church places hopeless alcoholics and the mentally ill. Robbie Mannheim is sitting with his aunt, Hanna, who is teaching him how to contact the "other world".

One day one of Robbie's classmates during school is severely wounded in the hand when a desk falls on top of him.

During one of Robbie's fits of hysteria, the parents find the letters SLU scrawled on his belly.

When McBride enters his room, he is attacked and becomes convinced that the child is endowed with some sort of supernatural power.

When Father Bowdern tries to convince the parents that there is nothing wrong with Robbie, the boy suddenly becomes hysterical, speaking in Latin as things fly across the room.

The Archbishop is skeptical, saying that he is trying to improve the Catholic Church's public image as a modern institution, free of ancient superstition.