Underlying the Basin and Range province of North America, giant caverns, some many miles in diameter, form a network that is occupied by people who have evolved the adaptations (such as pale skin) for living underground.

While exploring an abandoned silver mine in Nevada, Philip Clay and Frank Comstock are separated from their team and trapped by a cave-in triggered by a series of earthquakes.

Frank discovers that the vast caverns comprising the realms of Wu and Zu span hundreds of miles, primarily under Nevada, Utah, and Arizona.

Frank’s efforts to reform the society of Wu meet so much resistance and resentment that he is compelled to renew the war with Zu, whose own dictator has just been deposed.

The two men meet in secret and then, pursued by their erstwhile minions, they flee to a ventilation shaft where Frank has stashed climbing equipment.