The driver and fireman of a goods train are speeding along near the Scottish border. As they take a sharp bend, they can see that the track ahead has been ripped up. With no time to break, there’s no way to avert disaster. Before they know what’s happening, the engine has veered off the rails and plunged into the lake below, taking the wagons and men with it. Summoned north by the Caledonian Railway Company, Inspector Colbeck and Sergeant Leeming learn that the sabotage might be the work of a rival company. But as they proceed, a larger conspiracy seems to be afoot. The targets: the Queen and Prince Consort en route to Balmoral.