On the planet of Lithia, the terraforming has failed, and as that failed so did the colony. Some colonist families fled back to the space station. Most were left to struggle through on the planet's surface. As the ecology of the planet Lithia reasserts itself the atmosphere begins to revert to its natural state, and the flora and fauna follow suit. In increasingly unbreathable air and an increasing lack of Earth-natural food, those left on the surface sink into simple-minded savagery, with education far behind survival in their abridged priorities.

And there are the aliens, the orthong, wander and prowl the world.

Meanwhile in orbit around Lithia, Reeve Calder dreams of returning to the planet below him, and testing himself against the changing world and the deadly orthong.

He gets his wish sooner than expected. Shortly he'll down there on the surface. He'll be with Marie Dussault from Station, with Loon the strange local girl and Spar her bodyguard. Together they'll attempt to cross the massive and magnificent Rift Valley. Cross it they must, and quickly, if they are have any hope of saving this planet and their own lives.

Our various heroes struggle to prevent a planetary cataclysm, going through many, many trials and tribulations to reach their goal.