Boiled human bones have been found in Norwich's web of underground tunnels. When Dr Ruth Galloway discovers they were recently buried, DCI Nelson has a murder inquiry on his hands. Meanwhile, DS Judy Johnson is investigating the disappearance of a local rough sleeper, but with the discovery of the bones and the rumours of the chalk-mining tunnels being home to a community of rough sleepers, the clues point in only one direction. Ruth and Nelson must unravel the gruesome secrets of the underground before it claims another victim.