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.

Machi Kyouko is a dullahan, a demi-human with roots in Irish legend whose head and body are separate, and one of only three of her kind in the whole world. One day, she stays after school to talk to Tetsuo about herself. She explains that dullahan have unique problems of their own, and also asks a favor of him...

Yuki has become a bit more optimistic, thanks to Hikari and Machi. She's learned to trust Tetsuo a bit more since what happened last time, and has decided to tell her what troubles her about her own nature as a snow woman. It began shortly after she moved to a new city to start high school, when she was taking a bath one day and a shard of ice suddenly appeared in the hot water. It made her worry about what would happen if the cold air she generated were ever to be directed at a person, which led to her decision to avoid contact with others.

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!