One year ago, Nao was united with Shiro in Nikko. Unknown to him, Nao had another son—Masaki, her twin. Since birth, Masaki had been raised by a senior couple, and though he got along well with Shiro, he never knew they were brothers. One day, Masaki's adoptive parents revealed that Nao was his real mother, leaving him shocked. Learning that the aunt he deeply admired was actually his biological mother, his heart begins to stir. To help him accept the truth, Nao takes Masaki on a hot spring trip, embarking on a journey to close the emotional gap between them.