Shungo Ninomiya is a regular high school student who lives at home with his elder sister Ryoko—at least when she isn't gone on mercenary business. However, Shungo's world is turned upside down when a lovely girl called Mayu Tsukimura arrives in a military-grade chopper in the midst of his school's campus. Ryoko, it turns out, sent this girl and her elder brother Mikihiro to live with the Ninomiyas. But the freshly arriving siblings are a little different: Mayu is a succubus, and her brother is an incubus! As if the situation wasn't already complicated enough, Mayu suffers from androphobia (fear of males), and it's up to Shungo to assist her overcome this at the request of her father.