April, 1867. A train departs King's Cross station bound for Lincoln. Among the passengers is a man attired in the garb of one in high ecclesiastical office. He is accompanied by a brawny man carefully carrying a large leather bag, who ensures that they are not joined by any other travellers in their first-class compartment. Back at Scotland Yard that evening, Detective Inspector Colbeck is alerted to news of a brutal murder and robbery on the train. The urgent request for help from the Bishop of Lincoln states that a man has been shot dead and something of great value, a silver model of Lincoln Cathedral, has been stolen. Colbeck is more and more intrigued by the case as he and Sergeant Leeming follow the lines of inquiry, and he is convinced that the killer and thief is still close at hand. But with the city bursting at the seams for the annual Horse Fair, the timing is far from ideal and further complicates a challenging case for the Railway Detective.