Koichi, a son-in-law, lives with his wife and her parents. His father-in-law, who is unable to have children, interferes with his daughter and son-in-law's life by asking, "How is the process of having children going?" Koichi answers that he has no sexual desire, but the real reason is that he is fascinated by his mother-in-law Yumi's obscenely large breasts! Koichi's desire for Yumi reaches its limit, and he attacks her exposed large breasts while she is taking a bath! Koichi's courtship escalates as his reason is blown away, and Yumi, who is unable to use her ripe body, also becomes addicted to an affair within the family...