未来由于环境破坏等原因爆发了大洪水，大陆因此被水淹没，人类不得不生活在大海中。某日，少女艾尔菲发现自己在水中也可以呼吸，才知道原来自己是水栖人的孩子。水栖人强烈反对人类强行推进海洋开发的行为，与人类之间的对立逐步升级，促成了两军互相敌对，战争一触即发的状态。艾尔菲为了阻止战争在两股势力之间奔走斡旋。

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.