When an elderly lady is found inside her own house with her lungs full of river water, London's Peculiar Crimes Unit is called in to search for a logical solution. But the property's next tenant is plagued by mysterious damp patches and the sound of rushing water, and Arthur Bryant decides to divert the investigation into strange new territory. Meanwhile, his partner John May finds himself in hot water when he sets out to save the reputation of an academic whose arcane knowledge of London's forgotten underground rivers looks set to destroy his career. The geriatric detectives soon discover that in a city full of the rich, the poor and the dispossessed, there's still something for which a desperate murderer is willing to kill again. Pooling their information to investigate hitherto undiscovered secrets of the city, Bryant & May make some sinister connections. With the death-toll climbing and the unit facing an uncertain future, the nation's oddest investigators prepare to face madness, greed and revenge armed only with their wits, alcohol and boiled sweets in a mystery that goes to the heart of every London home...