Seventy-five years from now, the human race on Earth is in trouble - perhaps even facing extinction - because of the rapid evolution of diseases. A crew of young men and women travel to the moons of Saturn to investigate the biochemistry of the pre-life conditions there in the slim hope of discovering something that might save Earth. Nearly half of them have died on the way when the book opens, for they all have incurable diseases, doing most of their exploration with virtual-reality machinery. They race to find answers across the surface of an alien landscape with death close behind... and gaining.