Recently, my younger brother has begun to look at his father's erotic books and his mother's masturbation. Yuna, an older sister who couldn't see it, pays attention to her younger brother, but on the contrary, she feels sympathy for her wrong sexual knowledge. She started sexual education by expanding her genitals while being shy to her younger brother who seems to ask anyone about masturbation and sex, "It's really a secret of only two people."