After an unfortunate encounter with his new stepsister in the bathroom, Basara Toujou faces a challenging situation. His life takes a dramatic turn when his father introduces him to his new siblings, the Naruse sisters: the alluring Mio and the diminutive Maria. What initially appears to be a normal family dynamic transforms when Mio and Maria unveil their demonic heritage as the daughter of a former Demon Lord and her succubus aide. To complicate matters, Basara's own past as a Hero clashes with his newfound role as their protector. Despite the danger posed by both demons and Heroes, Basara forms a unique bond with Mio, pledging to safeguard her through a master-servant pact. As he navigates this treacherous path, Basara must confront not only external threats but also his own hidden demons, adding layers of complexity to his quest to shield his unconventional family.