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The Narumi family's lord, Tetsuo, was a strict father, and of course his eldest daughter, Urumi, and his younger brother, Ma-kun, were rigorously raised. One day, Ma-kun, who is beginning to be interested in sex, gets angry when his parents find a naughty book borrowed from a friend. Urumi, who sympathizes with such a younger brother, decides to give Ma-kun sex education with her own body.