When Basara Toujou's father suddenly brings home two beautiful girls and introduces them as his new siblings, he has no choice but to accept into his family the Naruse sisters: busty redhead Mio and petite silver-haired Maria. But when these seemingly normal girls reveal themselves as demons—Mio the former Demon Lord's only daughter and Maria her trusted succubus servant—Basara is forced to reveal himself as a former member of a clan of "Heroes," sworn enemies of the demons. However, having begun to care for his new sisters, Basara instead decides to protect them with his powers and forms a master-servant contract with Mio to keep watch over her. With the Heroes observing his every move and the constant threat of hostile demons, Basara has to do the impossible to protect his new family members. Moreover, the protector himself is hiding his own dark secret that still haunts him to this day...

Basara Toujou has a hard life. He is the older step-brother to two demonic sisters, Mio and Maria Naruse, whom he protects from entitled demons looking to claim Mio's power for themselves. On top of that, rising political tension within the demon realm only makes his job more difficult. When a messenger arrives with summons for Mio to the demon realm, she and her friends go in spite of the danger. Now on the enemy's turf, Basara will have to grow stronger through erotic pleasure in order to do the impossible and protect everyone.