Basara Tojo is a high school student at Hijirigasaka Academy when his father Jin unexpectedly "remarries" and asks him if he wants sisters. Basara meets his two beautiful and adorable stepsisters, Mio and Maria Naruse. Basara discovers immediately after his father left for overseas that the two sisters are actually members of a demon clan, and he is intended to be with the hero clan. Mio, a Demon princess, forcibly seeks to enter into a master and servant contract with Basara, but Maria has it turn out the other way around, with Basara as the master. Basara then continues to enter ecchi scenes, mostly to protect Mio from being followed by other demons who are pursuing her ancestors' inheritance, which is passed down from generation to generation.