Yumiko still has a good relationship with her husband, and their daily life has been going smoothly. However, recently their intimate encounters have decreased, leaving Yumiko feeling sexually frustrated. Around this time, she receives a balance ball as a gift from her son, prompting her to start exercising to improve her physical condition. She begins lightly working out once a week using the balance ball. But a week later, while exercising, Yumiko suddenly gets a leg cramp and calls her son for help. He rushes over and begins stretching her leg to relieve the pain. As he touches her sweaty body, he feels a rising arousal from her warmth. Yumiko too begins to feel stirred by the way he touches her. Though both pretend to remain calm, their awareness of each other gradually begins to shift...