Takahashi Tetsuo, a biology teacher at Shizaki High School, has been fascinated with "demi-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.
Machi Kyouko is a dullahan, a demi-human with roots in Irish legend, and one of only three of her kind in the whole world. One day, she stays after school to talk to Tetsuo about her unique problems, 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 him what troubles her about her own nature as a snow woman.
Keeping his promise to Himari, Tetsuo visits the Takanashi household to talk to the sisters' parents. Though the twin sisters' names differ by only one letter, Hikari is a vampire, while Himari is an ordinary human.
Sakie spots a beautiful blond-haired boy sitting on the floor, crying and hugging his knees. He says his name is Kurtz and doesn't seem to be a student of Shibasaki High. Meanwhile, Hikari says she's just seen another suspicious individual in the school.
Dullahans are demi-humans whose heads are separate from their bodies. Naturally, Tetsuo is intrigued by the unique traits of dullahans, but Kyouko is just as interested in learning more about her own body.
Tetsuo has offered to work with the girls to figure out how Hikari and Yuki can spend summer in comfort. But the school's head teacher suggests that the demi girls' total reliance on Tetsuo might actually be hurting them in the long run.
During summer break, Tetsuo asks Sakie to join him on patrol duty for the local summer festival, which both the demis and Himari and her friends go to. As each group decides to check out a nearby shrine, they get spooked by some strange occurrences.