In the 5th century, as Christianity gradually spreads across the Celtic lands and threatens ancient traditions, several monks are found murdered under mysterious circumstances. The evidence appears to point toward the druids, leading the monk Gwénolé to seek the help of Gwenc’hlan, one of the last representatives of the old religion, and his young apprentice Taran. Their investigation takes them across Armorica and the Celtic world, through legendary cities, ancient rituals, religious rivalries, and supernatural forces. As they search for the truth, Gwenc’hlan and Taran become caught in a conflict where the disappearance of druidic traditions and the rise of a new order seem increasingly inevitable.