The Special Assessment Unit. Sounds innocuous enough. But when you hear this unit of London's Metropolitan Police investigates the class of crimes codenamed 'Falcon', your ears might prick up. Dig a little deeper, and you might discover words like 'wizard' and 'magic' and 'little crocodiles'. Your conclusion might be that someone is taking the piss. But perhaps the investigations by Detective Constable Peter Grant - newly promoted cop and trainee wizard - and his colleagues at the Folly (HQ of SAU) - really are conducted in the shadowy infraworld of London's mythological past - and present. Among Peter's associates are the Rivers of London - the ancient gods and goddesses of the capital's waterways (one of them is also his girlfriend, Beverley) who, while often helpful, also skirt that fine line between good and, er, not so good. 'Rivers of London: Water Weed' takes place after the events of 'The Hanging Tree' and 'Cry Fox'.