Their legends tell of the Original World where man lived alongside the Light Almighty (a concept of which they can no longer conceive) and away from the ultimate evil, Radiation, with its two Lieutenants the Twin Devils Cobalt and Strontium.

He rationalizes that to find distant Light he must first locate its opposite, Darkness, which is near and "abounds in the worlds of men!"

By leaving the safety of the central echo-caster, with only a pair of click stones with which to listen, Jared exposes himself to soubat (once common cave bats that either "Cobalt or Strontium took ... down to Radiation and made [them] over into ... super-creature[s]") and Zivvers (people with an apparently unfathomable ability to navigate despite having poor hearing compared to the Survivors; it turns out that Zivvers navigate in darkness using infrared, while Survivors use echolocation).

Jared's quest for Light is interrupted by unexplained disappearances and an arranged marriage to Della, a girl from the Upper Level, the daughter of their chief 'the Wheel'.

Things get progressively worse as strange monsters roam the world and the hot springs begin to dry up.