A year ago, Nanao married her son Shiro in Nishi-Nikko.She actually had a son named Masaaki who is a twin brother to Nanao.Masaaki was raised by a senior couple from the time he was born, and although he was on good terms with Shiro, he did not know that they were brothers.One day, Masaaki learns from her foster parents that Nanao is her real mother.Masaaki is upset when he learns that Nanao, the aunt she loved so much, is her biological mother.In order to tell her the truth and have her accept her, Nanao decides to go on a hot spring trip with Masaaki.