Tetsuo, the head of the Narumi family, is a strict father, and of course Urumi, the eldest daughter, and her younger brother, Ma-kun, are brought up in a strict manner. One day, Ma-kun, who is starting to become interested in sex, is found by his parents with a dirty book he borrowed from a friend and is severely scolded. Urumi, feeling sorry for her younger brother, decides to use her own body to teach Ma-kun about sex...