Chisato Shoda and her son return to an old hot spring inn for a rare family visit. Her husband goes out for a walk, leaving Chisato and her son to relax together in the room. Lying down on the tatami, she gradually falls asleep. When she wakes up, she sees her son vigorously jerking off with a hard-on. Shocked and confused, Chisato struggles to react—but her son confesses, "I couldn't help it, Mom. Seeing you in that yukata turned me on. I've actually been attracted to you as a woman for a long time." From this moment, the story rapidly unfolds as Chisato faces an unimaginable dilemma.