Investigating the deliberate sinking of a number of oil tankers, Caroline Kent traces the trouble to a mysterious international company and its Chairman, Sir Edward Greatorex, but finds herself a prisoner in an underwater colony off the Bahamas, operated on to become one of a new species of aquatic human which Greatorex believes represents Mankind's future. His motives are altruistic, but nonetheless misguided and wrong; the subjects in his experiments don't have any say in the matter. While Caroline plots escape, her friends in the Security Services probe into Greatorex's activities. But as the net tightens around him, he resorts to increasingly desperate and dangerous means to protect his secret…