Eight years ago, Shinobu lost her husband in an accident and raised her son Yuuki single-handedly. Although they are mother and child, four years ago they had a physical relationship just once based on a promise. After that, they lived as ordinary mother and son, and eventually Yuuki got a job and moved out. Time passed, but Shinobu's heart was always haunted by the events of that day. One day, Yuuki invited Toshinobu on a hot spring trip as his final act of filial piety…
