High school student Basara Toujou’s life is turned upside down when his father suddenly remarries, introducing him to two beautiful new stepsisters, Mio and Maria Naruse. However, his new family dynamic takes a dark and dangerous turn when he discovers that Mio is actually the exiled demon lord’s daughter, possessing immense power that makes her a target for rival demon clans. To protect her, Basara forms a master-servant pact with Mio, but the ritual goes awry, inverting their roles and granting him dominion over her. As the trio navigates the treacherous politics of the demon world, they must also contend with the holy forces of the Hero clan, who are determined to eliminate Mio. This action-packed fantasy blends ecchi comedy with high-stakes battles, exploring themes of loyalty, family, and the struggle for power. Basara and his new sisters must forge an unbreakable bond if they hope to survive the escalating conflict between heaven and hell.