Shinichi has a wife and child, but he has special feelings for his mother. Three years ago, the two of them went beyond parent and child and made a vow to be a man and a woman for just one day... For a while, nothing happened between them. One day, Shinichi asked her to use his car to go shopping, and she gladly agreed, but he had secretly made a reservation for a hot spring inn deep in the mountains. The two could not forget the supreme moment they had three years ago... The babbling of the river in Oku-Nikko destroyed their reason and aroused their lust. And the two of them made the same mistake.