The book follows fourteen-year-old Will Burrows who lives in a fictitious London borough called Highfield with his family. Will has little in common with them except for a passion for digging, which he shares with his father. When his father inexplicably disappears down a tunnel, Will decides to investigate with his loyal friend, Chester. Soon they find themselves deep underground, where they discover the Colony, a subterranean city in which people have been living since Georgian times. These people have a deep hatred of anyone from the surface, who they call Topsoilers, and they are ruled by a strange and fanatical group called the Styx.