Nanase lived with her mother and her adolescent younger brother, Kousuke. Because of her mother, who was always busy with her job and didn't care about her family, Nanase took care of her housework and Kosuke. One day, Kousuke's crotch reacts to the bold appearance of her mother after taking a bath. Nanase, who knew her such a situation, decided to teach sex education with her own body because of her love for her ignorant brother.
