The protagonist is just a normal high school student living in a small town. However, something finally worth celebrating happens to him just before summer vacation: he had sex for the first time with two girls at the same time. The first girl is his older sister, Hasumi, while the other one is his childhood friend, Shizuno, who is the same age as him.
After receiving confessions from the two almost at the same time, the protagonist wondered for a while what to do -- until, a certain desire awoke within him, and he decided to go out with both of them. That desire is... sexually training the girls!
The blood-related older sister, who is refined and kind but shy. "I'd like make her into a pervy girl who loves sex." The childhood friend of the same age, who has tsundere tendencies and is stubborn. "Likewise, I'd like to make her into a pervy girl who finds sex pleasant."
With his girlfriends, he wants to have sex in all kinds of different ways. Thus, he decided it would be for the best to go out with the two of them at the same time.
So, what kind of results will the protagonist yield with his two girlfriends he intends to sexually train into sex-loving girls?