One day, the protagonist, "Takumi" witnessed it... His mother "Mayumi", and his friend "Eita" together, engaged in carnal pleasure. Fleeing from the sight of this, he runs into his friend's mother "Ayako", and together their relationship evolves into one between a man and a woman. Takumi, madly obsessed with this woman he has long since adored, is soon found out by his friend Eita. The two butt heads, and so they challenge one another, "Which one of us can truly prove they love their partner the most!?" And so begins the mother-son group sex competition...