The Morisawa household consists of a wife, her husband, and his father, living together peacefully. However, they face difficulties conceiving a child, and when the wife confides in her father-in-law, no solution presents itself. Noticing that her husband has many siblings, the wife suspects strong sperm runs in the family and decides to ask her father-in-law to impregnant her. Initially, their arrangement is purely transactional, but gradually, she becomes emotionally and physically drawn to him, falling deeper into their sexual relationship. They agree to end their affair once she becomes pregnant, and when she finally learns she's carrying a child, their intimate encounters come to a close. Yet after the baby is born, unable to resist their desire any longer, the two resume their breeding sessions once again...