Two nuns, Sister Junia and Sister Michaela, are confronted with a young boy named Hee-joon, who has been possessed by a powerful evil spirit. Sister Junia, driven by her faith and compassion, takes it upon herself to save the boy, while Sister Michaela, initially sceptical, is drawn into the investigation. As they delve deeper into the mystery, they encounter a series of chilling events and uncover a dark secret.