It is 1852, and Inspector Robert Colbeck and his assistant Sergeant Victor Leeming are faced with their most complex and difficult case to date. As a train speeds over the Sankey Viaduct, a man is hurled from a carriage and plummets into the canal below. It later transpires that he has been stabbed to death. With no papers by which to identify the man, the detectives' investigation is hampered from the start. Suspecting that the victim may have come from continental Europe, Colbeck and Leeming take the case to France where a new railway is being built by a British contractor. But in a new country the detectives face new problems. Anti-British feeling is rife and Colbeck and Leeming must put their own lives in danger to pick up the murderer's trail ...