Chimal is an Aztec farmer's son in an isolated valley cut off from the world by a landslide generations back, split between two villages that live under strict, ancient law enforced by priests. Unlike everyone around him, he keeps asking questions no one will answer about what lies beyond the valley's walls, curiosity that costs him an arranged marriage and, after a priest's death is laid at his door, very nearly his life. His escape, through tunnels no one admits exist, is only the beginning of what he finds out about his world. A standalone novel from Harry Harrison.
Summary by Loreful, from public sources