Basara Toujou's life takes a sharp turn when his father suddenly remarries and brings home two beautiful step-sisters, Mio and Maria.
However, these seemingly normal girls are actually demons: Mio is the daughter of a former Demon Lord, and Maria is her succubus servant.
Basara, who has a secret past tied to a clan of heroes sworn to defeat demons, chooses to protect his new family instead.
He forms a master-servant contract with Mio to keep her safe, placing him in a precarious position between his past and his newfound family, all while navigating the dangers of both the demon and hero factions.
Basara must learn to control his powers and protect his sisters from external threats while also guarding a dark secret of his own.