Earth's colonists have divided into the Swimmers, who spend their entire lives in zero-gravity and claim to be the next step in evolution, and the planet-dwelling Walkers. The Swimmers regard those who prefer to live on the surface of a planet as little better than unevolved apes, while the Walkers are not about to say farewell to the planets they grew up on, and think the Swimmers are not advanced at all, but merely deranged. Crowell, born a Swimmer but now a Walker by choice, is caught in the middle as the two sides prepare for war. Then he discovers the true cause of the altercation: a hidden alien race trying to provoke a war of extinction.