Eri, who lives in an ideal family, is treated coldly by her boyfriend for the unreasonable reason that she has too strong a sexual desire. At such a time, Eri's younger brother, Ma-kun, speaks kind words to her and cheers her up. Then, one day, when Eri sees Ma-kun's erect penis, she can't control her sexual urges and uses her own brother for sexual relief, claiming that it is sex education.