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.