Living with his wife and her parents, son-in-law Koichi finds himself caught between his childless marriage and the constant pressure from his in-laws. When his father-in-law bluntly asks, "How's the baby-making coming along?" Koichi lies, saying he has no sexual desire—when in truth, he's completely obsessed with his stepmother-in-law Yumi's obscenely large breasts. Unable to resist, he becomes fixated on her voluptuous figure, and his lust grows uncontrollable. One day, he makes a sudden move on Yumi while she's bathing. Overwhelmed by passion, Koichi's advances escalate wildly, breaking through all rational thought. Yumi, captivated by his raw desire, soon finds herself helplessly drawn into a secret affair, plunging both into a deep, forbidden incestuous relationship within the household.