Fifteen years ago, Sayuri divorced her husband and raised her son Tatsuya all on her own. Determined to spare him the hardships of a single-parent household, she worked tirelessly to support him through college and helped him secure a stable career after graduation. Grateful for his mother's unwavering love and sacrifice, Tatsuya dedicated himself to his job—commuting on the first train every morning, returning home on the last train at night, and rarely taking time off. One day, however, he suddenly collapsed from exhaustion. Worried and heartbroken, Sayuri decided to take him to a hot spring resort, hoping the relaxing environment would help heal his body and mind.