When rebellion destroys the underground world in which Thimble and Stone have grown up, they take Stone's son and try to escape the chaos. Along the way, they must evade the Freaks, beings who feed on human flesh. Leaving behind the roles of Builder and Breeder which they were assigned at birth, they wander the underground tunnels, looking for safety yet afraid to go "topside" where legend has it that the light and water will burn their skin from their bones.

Their journey takes them upward to an unimagined world of tinned food, comfortable furniture, and books. Away from their regimented society for the first time, and still facing imminent danger, Thimble and Stone acknowledge the forbidden attraction which both have denied for years.