Takahashi Tetsuo, a biology teacher at Shizaki High School, has been fascinated with "demi-humans," beings with characteristics that differ just slightly from humans, since he was in college. He wishes he could have a chance to talk with one, but since the demi-human population is fairly low, he's never even met one. But then the school gains a new math teacher, Satou Sakie... who happens to be a succubus.

Summer is finally here! Hikari pays another visit to the biology prep room to cool down, and while she's there, she asks Tetsuo for permission to reserve the school pool for a day. Ignoring Tetsuo's concerns that a vampire would struggle with the sun's direct rays in a pool, Hikari and the other demi girls meet at the school pool on Saturday for a day of play!