The head of the Narumi family, Tetsuo, is a strict father, and of course the eldest daughter, Urumi, and younger brother, Maa-kun, were brought up strictly. One day, Ma-kun, who is beginning to take an interest in sex, is scolded by his parents when they discover a naughty book he borrowed from his friend. Urumi, who sympathized with her younger brother, decided to use her own body to teach Ma-kun sex education.
