On the island of Erin, a rogue priest of the conquering Holy Empire abducts Riannon, the Gael tribe's young priestess, to sacrifice her in a ritual meant to resurrect the demon king Arawn and bind him to the priest's service. The ritual breaks Arawn's seal but leaves Riannon alive, and the Demon King who wakes proves far more human than the legends promised. Riannon and her older brother Arthur, the tribe's First Warrior, take him in, and the manga follows their growing band of fighters as they organize against imperial rule.