Yuki was born when Sayaka was 18 years old. Sayaka raised Yuki on her own. In order to make sure Yuki didn't have to endure the hardships of life, she quit college and worked late into the night, without any days off. Thanks to her efforts, Yuki was able to go on to college, but she always felt lonely. When Yuki turned 20, Sayaka decided to use up some of her accumulated paid leave, so the mother and son went to a hot spring inn. However, all she could think about was work.