While on their quest to stop the Demon King, the violinist Hamel, the pianist Raiel, the magess Flute and the female warrior Sizer stop in a small kingdom and destroy a rampaging monster. While giving the four a heroes' welcome, the king and queen explain that their daughter has been kidnapped by a monster living in a nearby castle. They agree to go and rescue the hostage princess and defeat the monster. However, the enemy's castle is heavily guarded...
In the future, the oceans have risen to flood all the continents due to humanity's negligence of the environment. Only a few bits of land have been spared, and one of those is the city-island where Elfie lives with her grandfather. One day, due to an underwater incident, she discovers that she can breathe underwater. Her grandfather reveals that in truth she is one of the mythical sea-people. Using the city folk's xenophobia, local politicians spark a war with this hidden people to distract people from their current resource problems, and Elfie is caught in the middle of it.