Chimal lives in an Aztec valley, where the serpent-headed goddess, Coatlicue, walks. When Chimal notices that all creatures entering the valley seem to come from the same place, he goes to investigate. Leaving the valley, he discovers that it is contained in a 'generation starship', which is run by a group who live lives similar to those of medieval monks.

Chimal first adjusts to life outside the valley, then sets about liberating the inhabitants of the ship.

This is regarded as one of Harrison's most literary novels.