The action takes place in the far future on the planet Undine, a water world of vast oceans dotted by islands. An eternally youthful Empress rules a federation of loosely linked planet-nations from the Imperial Planet, the largest of the Empire's Seven Worlds. It is the time of the yearly Fish Festival when the double moons merge and the Undians, both human and Gaxulta (a being transformed to live under water), take a few days to celebrate their oceanic livelihood. All sorts of non-violent behaviour is tolerated. Personalities conflict, telepathic powers are abused and danger looms. Life beneath the water offers an attractive freedom, however not without a price; it represents the loss of family, friends and the effects of civilization.
Gaxulta (the word refers either to an entire community or to a single member) are not born; they portray those that have chosen to endure painful physical change accomplished through the discipline of the mind. They learn to cultivate and control life's gifts that are treated censoriously on land.