Roy Complain lives in a culturally-primitive tribe in which curiosity is discouraged and life is solitary, poor, nasty, brutish and short. With a small group, he leaves his home and ventures into uncharted territory. The consequent discoveries will change his perception of the entire universe.

Complain's small tribe roam nomadically through corridors overrun by vegetation. After his wife is kidnapped, a tribal priest named Marapper encourages Complain to join a furtive expedition into the unexplored corridors. It is Marapper's belief that they are all living on board a moving spacecraft and that if they can reach the control room, they will gain command of the entire gargantuan vessel.

On their journey, the group encounters other tribes of varying levels of sophistication. Complain is also briefly captured by humanoid 'Giants' of legend, who release him with no explanation. Complain's party eventually join the more sophisticated society of the 'Forwards'. Here, they learn that the space-craft is a multi-generational starship returning from the newly colonized planet of Procyon. In a previous generation, the ship's inhabitants had suffered from a pandemic because of an alien amino acid found in the waters of Procyon. Law and order began to collapse and knowledge of the ship and its purpose was eventually almost entirely lost throughout the vessel. It is now 23 generations that have passed since this 'Catastrophe'.