In the early hours of August 8, 1963, a Royal Mail travelling post office runs from Glasgow to London carrying a haul later valued at over £2.6 million. Fifteen men, led by Bruce Reynolds and acting on a tip from an inside source known only as "The Ulsterman," rig the trackside signals to stop the train at Sears Crossing, then storm the mail cars. Driver Jack Mills is beaten with a metal bar badly enough to end his career, one of the few violent moments in a job otherwise built on planning rather than force. The gang scatters to a hideout at Leatherslade Farm, where fingerprints and supplies left behind hand police the evidence that unravels the operation and sends the ringleaders to prison with thirty-year sentences. The film follows the raid, the manhunt, and the trial in sequence, using the case to trace how a crew of small-time criminals pulled off one of the largest cash robberies in British history, and how thoroughly they were caught.