When Yuuki was born, his father had already disappeared. Despite strong opposition from those around her, Hotaru gave birth and raised Yuuki all on her own, enabling him to eventually enter university. Because they were close in age, their relationship had always felt less like mother and son and more like friends. One summer vacation, Yuuki invited Hotaru on a hot spring trip. While Hotaru looked forward to a carefree getaway, Yuuki had a special feeling he'd wanted to confess to her since becoming an adult. He planned this trip as a chance to finally share that emotion with her.