Mika Miko Shiba's husband still has a strong sex drive, and they have sex almost every night. Lately, however, he finishes in just one round, leaving her unsatisfied and frustrated. Feeling increasingly restless, she decides to take matters into her own hands and starts exercising to relieve her tension. When she discusses this with her son, he offers to gift her a balance ball, and from then on, she begins using it daily for workouts. One day, she gets a leg cramp, and her son notices, offering to help stretch her legs. As he assists his mother with stretching, he begins to feel an uncontrollable arousal. Mika also starts feeling strangely excited by her son's touch. After each stretching session, her body grows hot and her emotions become unsettled. Eventually, she begins secretly masturbating to cope. But her actions are discovered by her son, and the story takes a new, intense turn.