Abandoned as a teenager on a strange planet, Casp Alkin earns a living transporting cargo through the ancient tunnels which lie beneath the rapidly industrialising Liquid City. It's a dirty, dangerous game but one which affords them the independence they always longed for.
But the offer of a lucrative contract persuades Casp to break with their established routine and journey into the unexplored parts of the tunnel network, deep underground. Accompanied by an ill-tempered cephalopod and the scientist daughter of their wealthy sponsor, Casp embarks on an increasingly dangerous journey. But they quickly find that success will mean an end to the livelihoods of the tunnel folk who have become their family, destroying the community which has relied on the tunnels for generations.
Casp has little hope of saving the tunnelling industry, but they are persuaded to take to the tunnels one last time. Because some believe that humans weren't the first to inhabit this isolated planet, and what they find will shift the balance of power in Liquid City forever.