In an alternate Australia dominated by coal power and the British empire, Clara and Tim arrive in the rebel republic of Westralia, where people are nocturnal and live underground, burrowing with machines called steam moles.

Sending a spaceship to rendezvous with an enormous vessel made up of a series of globular habitats approaching its star system, the aliens find a savage race calling themselves "humans" who promptly slaughter the exploration team, except for a single survivor who escapes to another habitat, where he finds more friendly humans whose help he requires to get back to his own ship.

After eliminating the Worm World threat and outwitting a deadly gang of space pirates, Captain Pausert embarks on a mission for the Empress Haile to stop a nanite plague before it can devastate entire worlds.