There have been ghosts on the London Underground, sad, harmless spectres whose presence does little more than give a frisson to travelling and boost tourism. But now there's a rash of sightings on the Metropolitan Line and these ghosts are frightening, aggressive and seem to be looking for something.Enter PC Peter Grant, junior member of the Metropolitan Police's Special Assessment unit a.k.a. The Folly a.k.a. the only police officers whose official duties include ghost hunting. Together with Jaget Kumar, his counterpart at the British Transport Police, he must brave the terrifying crush of London's rush hour to find the source of the ghosts.Joined by Peter's wannabe wizard cousin, a preschool river god and Toby the ghost hunting dog, their investigation takes a darker tone as they realise that a real person's life might just be on the line.And time is running out to save them.With this new novella, bestselling author Ben Aaronovitch has crafted yet another wickedly funny and surprisingly affecting chapter in his beloved Rivers of London series.