The brother and sister meet again for the first time in several years in the countryside in midsummer. Nobu, who is very interested in erotic things, makes a suggestion to Kouji. "I'll let you rub my sister's breasts, so let me rub your sister's breasts." They immediately put it into action. Then Yukino says, "Have you all ever had sex? It feels really good." With that one word as a trigger, they had sex for the first time under the pretext of practicing. And so, the four teenagers discovered pleasure and spent their days having sex whenever they had the chance, like monkeys.