Captive Universe

Captive Universe is a 1969 science fiction novel by American author Harry Harrison. Its protagonist, Chimal, is a young Aztec tribesman from the village of Quilapa, living in a valley sealed off from the outside world by an ancient earthquake. Unlike his fellow villagers, Chimal is unusually intelligent and curious about what lies beyond, which marks him as strange and often gets him into trouble. The valley contains two villages—Quilapa and Zaachila—separated by a river, sharing a temple where priests enforce the gods' laws, including a strict ban on intermarriage between the two communities. Chimal’s mother, Quiauh, conceals the fact that she conceived him with a man from Zaachila.

When Chimal refuses an arranged marriage, the chief priest, Citlallatonac, dies of a stroke during the ensuing argument. Chimal is then condemned to sacrifice. The gods appear angered, and the sun fails to rise at dawn. In the panic, Quiauh rescues Chimal by taking his place in the cell, revealing his true father’s identity as she does. Escaping, Chimal evades the priests and the mechanical serpent that hunts at night near the river. He eventually breaks through the blockage at the valley’s end, discovering a series of tunnels. There he finds the serpent deactivated and harmless, and later encounters a woman named Watchman Steel, whom he forces to guide him further.

Chimal meets the Master Observer, who hails him as the "First Arriver." He learns the valley is the central cavity of a generation ship. Five hundred years earlier, an Earth dictator known as the Great Designer converted the asteroid Eros into a spinning starship, sending it to Proxima Centauri. To maintain order over centuries, the passengers were split into two groups, each genetically engineered for low intelligence in different ways and given Aztec culture. The Observers, also programmed, were given a monastic culture to tend the ship. On Arrival Day, the two Aztec groups would interbreed, removing the recessive traits and producing intelligent offspring to colonize the new system.

However, Chimal discovers a crisis: the Observers, too rigid to interpret planetary data, have overridden the flight plan and passed Proxima Centauri. When he tries to explain, they denounce him as a heretic. After a pursuit through the ship, he activates the Arrival program, breaking the barrier between the two Aztec communities.