Haruka is an older sister who dotes on her younger brother. However, one day her younger brother starts saying "Let's have sex!" in front of the family over and over again, surprising them. Haruka is troubled and investigates the cause, and finds that it seems to be influenced by the fact that her younger brother's male friend is having sex with their mother. Haruka that her younger brother is innocent about sex, Haruka decides to instill proper sexual knowledge in him...