When Kepler Masterman, the first child born and reared on Moon Colony, comes to Earth for a six-month stay he's unable to take the gravitational pressure and is sent to live on Conshelf Ten, an undersea community engaged in fish-farming and hydroponics. Kepler loves the weightlessness of scuba-diving but soon finds himself caught in a web of conspiracy. Conshelf, like Moon, is fighting for its independence, and Kepler discovers that Hilary, his friend and teacher, is an advocate of radical terrorism. Stranger still, he encounters a gillman, a human being surgically adapted to underwater life, although others on Conshelf insist that ""the gillers"" are mere legend. There's a good deal of naive plot (Hilary is later forgiven for planning revolution) and Kepler's efforts to persuade the gillers to abandon violent tactics are leaden. But Hughes offers persuasive glimpses of undersea living--from the technology of Conshelf's pressure-sealed domes to a giller's anguished explanation of the psychological isolation of living in an alien body and environment.