Son Keiichi can't even stand seeing Mari and Hiroki being affectionate. He is a serious and fastidious person. Mari is worried about such a Keiichi. One day, Mari catches Keiichi masturbating and thinks, 'Maybe he's just nervous around women?' So she helps him with his masturbation. Keiichi, who has lost control, doesn't care about the situation and pounces on Mari. However, Keiichi's desires are insatiable, and Mari herself also gets used to the relationship, becoming hooked on Keiichi's skilled techniques as he eagerly studies sex...
