Underwater kingdom Kayen venerates the Water Dragon, the guardian god. Once people wandered the world in search of a peaceful land, and the Water Dragon granted them a peaceful life and a land where there was no room for conflict. However, those who wished to disrupt this peace appeared, so the priestess named Yura, who prays to the long Sondrakon that has plunged into the depths, is forced to go to the surface. At that moment, the Tempest festival marking the opening of the country had just ended, and Rimuru and his companions decided to treat themselves to a short, somewhat unpleasant vacation. Now, in order to save Yura, Rimuru and his companions must dive again.