Chimal is a young Aztec tribesman living in an isolated valley which was sealed off from the rest of the world in ages past by a massive earthquake.

The Gods, angry at the sacrilege, cause the sun to fail to rise at dawn, and in the panic, he is rescued by his mother, who takes his place in the cell where he is held.

Escaping, he attempts to find a way out of the valley and to avoid being hunted by the priests, and Coatlicue, the dreaded two-headed serpent-Goddess who stalks the land at night and kills those who foolishly wander near the river past sunset.

The Aztecs are its passengers, tended by the ship's crew in the caverns, the Observers, on a centuries-long flight to Proxima Centauri, the nearest star to the Sun.

Five hundred years ago, "The Great Designer," an Earth dictator, converted the asteroid Eros into a generation starship, spinning along its axis for gravity, and sent it on its journey.

On Arrival Day, the two Aztec groups would be allowed to breed together, which would remove the artificial recessive traits and produce highly intelligent offspring able to colonize the new system.