Boiled human bones have been found in Norwich's web of underground tunnels. When forensic archaeologist Dr Ruth Galloway discovers the bones aren't as old as originally thought, it's time for DCI Nelson to launch a murder inquiry. What was initially just a medieval curiosity has taken a much more sinister nature....
Meanwhile, DS Judy Johnson is investigating the disappearance of a local rough sleeper. The only trace of her is the rumour that she's gone 'underground'. This might be a figure of speech, but with the discovery of the bones and the stories both Ruth and the police have heard of a vast community of rough sleepers living in the old chalk-mining tunnels under Norwich, the clues point in only one direction. Local academic Martin Kellerman knows all about the tunnels and their history - but can his assertions of cannibalism and ritual killing possibly be true?
As the weather gets hotter, tensions rise. A local woman goes missing and the police are under attack. Ruth and Nelson must unravel the dark secrets of The Underground and discover just what gruesome secrets lurk at its heart - before it claims another victim.