16-year-old Susan Garth lives with her single father. The two of them attend a Catholic Church regularly every Sunday and are considered reputable citizens by all of their neighbors. When Susan begins to suffer from unexplained seizures and becomes difficult to manage, she refuses to return to church. Right around this time two things happen. Father Gregory Sargent becomes the priest of their local parish. And he is visited by his friend, Bishop Crimmings. Desperate for some help with his increasingly difficult daughter, Mr. Garth appeals to Sargent for help.
At first, they dismiss Susan's bizarre behavior as some kind of teenage fit, but Sargent and Crimmings soon change their tone as it becomes increasingly more clear that she is truly disturbed. Susan starts cursing regularly, especially at the priest and the bishop. In one incident she strips naked before the priest and attacks him. In their investigation, the church leaders discover that Susan has good reason for her mental disquiet. Her father disciplines her by beating her and listens to the advice of a friend who vehemently decries the Catholic faith and blames the girl's condition on her religion. Needless to say the relations between the dad and the priest become hostile. As the case progresses, Russell introduces many elements which beg readers to decides for themselves whether Susan is responsible for her actions or whether she's been possessed by a demon.
Bishop Crimmings encourages Sargent to reconsider his religious beliefs about evil. He warns his inferior that often religious people have focused so much on the gentle, loving aspects of God that they've forgotten about the selfish, evil powers which oppose him. The story ends up being about Sargent and his journey of faith which is tested by Susan. Examining the various elements of the case, he excuses her sexual promiscuity as religious zest and connects her seizures to extreme mental duress, but he still cannot decide as to whether she's actually possessed or not. In the end, he is forced to reincorporate evil into his worldview, but it comes as a surprise. He does not pass judgement on Susan, but he doesn't believe that she's possessed. Much to Crimmings's satisfaction (and horror), the priest concludes that real evil is present in all human beings as a result of their fallen nature.
