When an older brother, who is worried that he will never be able to get a girlfriend because of his foreskin, makes fun of his younger sister, who is worried about her small breasts, she strongly retorts, "My condition is more serious than yours!" They decide to show each other evidence and flaws rather than arguments. Although he said it on impulse, the older brother ends up getting erect when he sees his sister's breasts. Furthermore, the younger sister's erect penis makes her horny... They can't control themselves and what was supposed to be a sex consultation turns into incest!