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 enquiry on his hands. The boiling might have been just a medieval curiosity - now it suggests a much more sinister purpose. 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 rumours both Ruth and the police have heard that the network of old chalk-mining tunnels under Norwich is home to a vast community of rough sleepers, 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.