There's something going bump on the Metropolitan line and Sergeant Jaget Kumar knows exactly who to call.
It's PC Peter Grant's speciality . . .
Only it's more than going 'bump'. Traumatised travellers have been reporting strange encounters on their morning commute, with strangely dressed people trying to deliver an urgent message. Stranger still, despite calling the police themselves, within a few minutes the commuters have already forgotten the encounter - making the follow up interviews rather difficult.
So with a little help from Abigail and Toby the ghost hunting dog, Peter and Jaget are heading out on a ghost hunting expedition.
Because finding the ghost and deciphering their urgent message might just be a matter of life and death.

Before becoming a bestselling author, Ben Aaronovitch was a screenwriter for Doctor Who and a bookseller at Waterstones. He now writes full time, and every book in his Rivers of London series has been a Sunday Times Top Ten bestseller. He is published in 14 languages and has sold more than 2 million copies around the world. Aaronovitch is also a trustee on the board of Cityread London and is a long-time supporter of Nigeria's premiere arts and cultural festival, The Aké Festival. He still lives in London, the city he likes to refer to as 'the capital of the world'.