Shinichi has a wife and children, but his mother has a special feeling. Three years ago, the two had transcended their parent-child relationship for just one day and made a pact as a man and a woman. The two of them had nothing to do for a while. One day, Shin'ichi wanted to go shopping, so he kindly accepted the offer, but secretly made a reservation for a hot spring inn deep in the mountains. The two couldn't forget the supreme time three years ago... The babbling of the river in Okunikko breaks reason and drives lust. And the two of them end up making the same mistake.