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!

Tetsuo has offered to work with the girls to figure out how Hikari and Yuki, who don't handle heat or sunlight well, 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. This weighs on Tetsuo's mind...

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 the idea to ask the opinion of someone outside of his own background in biology, so he takes Kyouko to meet Souma, an old college buddy.

Sakie the succubus is spending her evening at home alone, ranting her troubles to her teddy bear, when she receives a call from Ugaki. Sakie knows she can't avoid the influence of her inborn aphrodisiac effect on romantic relationships, but she doesn't want to use it in the process of starting a relationship... so Ugaki offers her some advice.

The midterm exam score rankings are posted. While Himari and Kyouko earned high ranks, Hikari is scolded by Tetsuo for getting three failing grades, so she resolves herself to study along with Yuki. Meanwhile, Yuki drops something important to her somewhere in the school, and Sakie happens to find it, but Yuki has trouble admitting that it's valuable to her...

As she's walking through the school building, 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, behaving a bit suspiciously. Meanwhile, Hikari says she's just seen another suspicious individual in the school, describing him as a large man who looked like a bear...

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. Though they argue often, they are very close. When he experiences the warmth and kindness of the family that raised these twin sisters, Tetsuo stays much longrer than he had intended.

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.