In April 1851, shortly before the opening of the Great Exhibition at the Crystal Palace in Hyde Park, London, a mail train on the London and North Western Railway (L&NWR) is halted between Leighton Buzzard Junction and Linslade Tunnel by a group of men disguised as railway police. Using duplicated Chubb safe keys, they steal all the mailbags and a consignment of over £3,000 in sovereigns being transferred from the Royal Mint to a bank in Birmingham. The train driver, who tries to resist the robbers, is badly injured and his fireman is forced to drive the engine forward to where a section of track has been removed, causing a derailment. The robbers escape and the alarm is raised by telegraph to the Metropolitan Police Force in Scotland Yard, London, where it is received by Detective Superintendent Edward Tallis, head of the Detective Department.
Tallis summons Detective Inspector Robert Colbeck to his office and orders him to lead the investigation. It is quickly established that there is an "unresolved tension" between Tallis and Colbeck which is an underlying theme of the whole series, although the two have great professional respect for each other. Colbeck and Detective Sergeant Victor Leeming travel to Leighton Buzzard where they meet Inspector Rory McTurk of the railway police. McTurk strongly resents their involvement but Colbeck and Leeming soon establish that members of the railway police were guilty of deserting their posts on the train in order to play cards, thus making it much easier for the gang to carry out the robbery. Meanwhile, train driver Caleb Andrews is not expected to survive and his daughter Madeleine arrives in Leighton Buzzard to care for him.
Colbeck is convinced that the gang had inside information and were certainly in possession of duplicated safe keys. He and Leeming begin enquiries at the organisations involved with the shipment: the L&NWR, the Post Office, the Royal Mint, the fictitious Spurling's Bank in Birmingham and the Chubb factory in Wolverhampton. As a result, they become aware of Daniel Slender, who duplicated the safe keys at Chubb; William Ings of the Post Office, who was involved in the mail train's scheduling; and Albert Woodhead of the Royal Mint, who unwittingly advised Ings of the gold consignment. The L&NWR and the Birmingham bank are absolved of involvement in the crime.
Ings is the first of the suspects to be pursued and Colbeck discovers that he is hiding in the notorious Devil's Acre. To find Ings, Colbeck seeks help from Brendan Mulryne, a former colleague who is now a resident there. Mulryne establishes a connection between Ings and the prostitute Polly Roach but, though he finds Roach, he is too late to prevent the murder of Ings. In due course, Slender meets a similar fate. It is clear to Colbeck that the gang are eliminating loose ends and weak links to remain undetected. Colbeck is concerned about the unnecessary destruction of the steam engine after the robbery and believes he is looking for someone with influence who hates the railways.
Although Tallis dismisses Colbeck's theory, it is valid as gang leader Sir Humphrey Gilzean has an obsessive hatred of railways. He blames them for the accidental death of his wife. The gang strikes again at the Kilsby Tunnel, placing gunpowder at one end in a bid to destroy both the tunnel and a train carrying glass sheets for use in the Crystal Palace. The explosion is detonated too soon to destroy the train and causes only superficial damage to the tunnel. Colbeck makes the connection and realises, despite more opposition from Tallis, that an attempt will be made to destroy the locomotives on display at the Great Exhibition.