Basara Toujou's introduction to his new step-siblings did not pan out as he had hoped. His father brought home two beautiful girls, Mio and Maria Naruse, and introduced them as his new sisters. However, much to Basara's shock, they were not ordinary girls but demons. Mio was the daughter of the previous Demon Lord, while Maria was her succubus servant. As a former member of the "Heroes" clan, who were sworn adversaries of the demons, Basara was in a tricky situation. Nevertheless, his love for his new siblings compelled him to safeguard them, prompting him to form a master-servant contract with Mio and utilize his powers to keep an eye on her. However, protecting his family won't be a cakewalk, given the constant threat of hostile demons and scrutiny from the Heroes. Furthermore, Basara had a dark secret that continued to haunt him.