13 cute little sisters who were teased in the bath. An older brother takes advantage of the absence of his parents, who feel safe with their siblings, to play pranks on his younger sister. To an older brother who is horny, his younger sister is nothing more than a tool for masturbation. "If you don't wash your private parts properly, germs will get in, so I'll wash them for you, okay?" The escalating pranks finally develop into forbidden incest.