Teenager Hideki Nishimura plays a massively multiplayer online role-playing game called Legendary Age (LA), but one day he proposes online to a girl who tells him she is really a man, so he swears off online marriages. Two years later, he has been involved with a guild and eventually accepts an in-game marriage offer from his persistent guildmate, Ako

"You wanted little sisters? Congratulations—you just got a Demon Lord’s heir and her succubus!" High school student Basara Tojo’s life is turned upside down when his father’s sudden remarriage drops two adorable stepsisters into his lap: Mio and Maria Naruse. But the moment his dad departs on an overseas trip, the sweet front completely crumbles. Mio isn't an ordinary girl—she’s the daughter of the late Demon Lord Wilbert, and Maria is her loyal succubus bodyguard. Worse yet, Basara is hiding a lethal secret of his own: he’s an outcast from the rival Hero Clan. Determined to take control, Mio attempts to force Basara into a Master-Servant magic pact. But when Maria's interference accidentally reverses the spell, Basara finds himself the Master of a Demon Princess! Now, with hostile demon factions hunting Mio down for her inherited power, Basara must risk everything to protect her—even if reinforcing their magical bond forces him into one outrageously steamy situation after another.