After losing her husband, she raised her son alone. He's about to graduate from university. However, Yuria had a secret worry. A few years ago, she had a one-time physical relationship with her son. One day, they decided to go on a hot spring trip together for his last memory as a student. Memories of that day their relationship crossed a line resurface... Sharing a hot spring bath, staying in a private room, even though they know it's wrong, they gradually lose their reason and their instincts take over, leading to a forbidden incestuous tale as they become intertwined.