Yumi was happily married to her divorced husband and they lived together in harmony. Their sexual life was also fulfilling, both in terms of frequency and quality. However, Yumi felt a sense of dissatisfaction with the size of her husband's manhood. One day, her husband's son from his previous marriage was scheduled to come and stay with them for a while due to work, and they would live together as a family of three. Yumi was delighted to welcome him, whom she had always treated like her own son. When the doorbell rang and she opened the door to let him in, she was slightly pleased to see that he had grown up a bit. At night, Yumi woke up to a suspicious sound. When she followed the sound, she found it was coming from his room. She peeked in secretly and saw him in the middle of a masturbation session. Just as she was about to turn back, she was shocked. His son, who seemed about to ejaculate, was much larger and more robust than she had ever seen before. Yumi couldn't take her eyes off his actions for a while, but then she suddenly came to her senses and quickly returned to her room. In the morning, after seeing her husband and his son off to work, Yumi busied herself with housework. However, the sight of his actions she had accidentally witnessed kept coming back to her mind, making her body gradually warm up, and her fingers began to naturally stimulate her most sensitive area...