Marseguro, a water world far distant from Earth, is home to a small colony of unmodified humans known as "landlings" and to the Selkies, a water-dwelling race created by geneticist Victor Hansen from modified human DNA. For seventy years, the two groups have lived in peace. Now a vengeful landling has activated an old distress beacon that will bring down forces from Earth who are dedicated to restoring genetic and religious purity and will try to eradicate the Selkie race entirely.