"If you can't have sex with a woman, then there's no point in living as a man." These are the words that our protagonist, Yashiro Kei, heard too often from his handsome brother, Ei.

In the spring of his second year―― One day after school, Kei witnesses a shocking scene involving his childhood friend who became estranged from him due to certain circumstances, Mitsubuchi Yua, and a half-Japanese beauty in the same class who is the chairman's daughter, Takayashiki Jane Serika.

Because he sees the female homeroom teacher, Amano Kusuha, having sex with Yua and Serika. After unconsciously taking pictures with his smartphone, Kei leaves the scene. However, Kusuha finds out about his peeping, and the next day after school, he receives a call.

"I don't want to get expelled," Kei cries, so Kusuha gives him a condition. He has to join the "Sex Friend Club" that she has secretly created, and serve as a partner of Yua and Serika.

Thus, the ordinary school life for Kei comes to an end, while his extraordinary school life begins──