Nanase lived with her mother and her adolescent brother, Kosuke. Because she was a mother who only worked and didn't care for her family, Nanase was also taking care of housework and Kosuke. One day, Kosuke's crotch reacts to the appearance of a bold mother after taking a bath. Knowing such a situation, Nanase decides to teach sex education with her own body because of her love for her ignorant younger brother.