In a quiet and remote convent in South Korea, a group of nuns lives under the strict watch of the mother superior, who has kept a dark family legacy a secret for centuries. With the arrival of a new postulant, their lives take an unexpected turn. The young woman, with a tumultuous past and a rebellious spirit, begins to uncover mysterious letters hidden in the convent's library and a series of rituals that seem to be connected to ancient legends of witchcraft.