When the number of possession cases increased dramatically, the Vatican began to open exorcism schools around the world, where they not only teach how to cast out the devil, but also observe the patients to see if they are really possessed by evil forces or if they are mentally ill. A young nun, Ann, cares for such people and is eager to learn, since years ago her mother was killed by a demon who possessed her. Despite the protests of the older nun, the girl gets permission to attend the lectures of the experienced exorcist Father Quinn and even shows herself in practical classes, but she does not realize yet how cunning demon she will soon have to fight.