Shinichi is a married man with a wife and child, yet shares a special bond with his mother. Three years ago, the two crossed the boundary of parenthood and engaged in a fleeting intimate encounter—an unforgettable moment that would bind them forever. Though they continued life as if nothing had happened, one day Shinichi asked her to drive him to run errands. Unbeknownst to her, he had secretly reserved a secluded hot spring inn deep in the mountains. The sound of the flowing river stirred their emotions, leading them to repeat the same forbidden act once again.