Basara Toujo's father remarries and leaves him with two new step-sisters: Mio Naruse, daughter of the former Demon Lord, and her succubus attendant Maria. Basara himself is an exile from the Hero Clan, cast out after losing control of his own power, and he's barely settled into his new family before rival demon factions and his old Hero Clan start hunting Mio. Maria's solution is a Master-Servant contract binding Basara and Mio together, a succubus's magic that comes with plenty of ecchi complications. This original light novel is the source for the manga and anime.