One day, the overly serious Hoshizora-chan was secretly worried that she couldn't keep up with her classmates' conversations about sex. She decided to go to her PE teacher's house on a weekend to ask him to teach her about sex. However, the teacher was a genuine lustful teacher who visited a brothel every week. At first, he tried to get rid of Hoshizora-chan who had come to his house to go to the brothel, but he took advantage of her worries and started playing with her body, which was still in the process of development. The serious Hoshizora-chan also took what the teacher said seriously, and gradually her body and Kokoro were dominated...