Basara Toujou learns the hard way that running into your new stepsister in the restroom is not the greatest way to make a good first impression. When his father unexpectedly brings home two gorgeous girls and presents them as his new siblings, he is forced to welcome the Naruse sisters, busty redhead Mio and diminutive silver-haired Maria, into his household. Basara is compelled to disclose himself as a former member of a clan of "Heroes," sworn opponents of the demons, when these seemingly normal girls show themselves as demons—Mio, the previous Demon Lord's sole daughter, and Maria, her faithful succubus servant. Basara, having begun to care for his new sisters, instead resolves to defend them with his abilities and creates a master-servant relationship.