Kaien is an undersea nation whose people worship the Water Dragon as their guardian deity. Their ancestors lived among other clans on the surface long ago, but after wandering the earth in search of a land of peace and stability, they were blessed by the Water Dragon with a kingdom free of conflict. However, that peace would not last forever. Yura, the priestess who prays to the long-dormant Water Dragon, learns that someone is plotting to invade the surface in order to wake the Water Dragon, so she takes the flute that has been passed down in her clan for generations and heads for the surface to seek aid. When she arrives, she finds herself on a resort island in the domain of Elmesia, empress of the Sorcerous Dynasty of Thalion, and she meets Rimuru and his compatriots, who are enjoying a brief vacation after concluding their Nation's Opening Festival. Rimuru’s gang accepts Elmesia’s request to accompany Yura to Kaien so they can help, but a conspiracy is already brewing under the sea... After the Water Dragon is awakened and turmoil breaks out over the flute, Yura’s hidden power comes to light. Can Rimuru and company protect the azure sea from the approaching threat and restore peace to Kaien?