Mikoto and Takeru love their friend Tachibana, but Tachibana has a terrifying secret hidden inside him: a demon of monstrous evil waiting to be set free! That's not the sort of thing that can be cured with an over-the-counter medication, but Hatsune, the girls' mother, has a plan that might be able to trap the demon forever. The catch is that Hatsune must have sex with Tachibana, and then one of her daughters must sleep with him as well, thereby conceiving a new demon to counter the first.