Twenty years after her divorce, Chizuru Iwasaki raised her son alone. One day, when her son came to her with a sexual question, she realized how lonely and ignorant he was. Since then, Chizuru began seeing her son as a sexual partner, and their relationship naturally progressed into a physical one. Now, this relationship continues as a matter of course, and Chizuru secretly wonders if this is really acceptable. Yet she knows she was the one who initiated it and bears responsibility. Hoping to broaden her son's horizons, Chizuru decides to invite him on a hot spring trip.