About two centuries 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 Titan, to investigate the biochemistry of the pre-life conditions there in the slim hope of discovering something that might save Earth. Nearly half of the crew die on the way. They have to do most of their exploration in virtual-reality machinery. The whole story runs at high speed, as they race to find answers across the surface of an alien landscape with death close behind, and gaining.