November 1907. George Dillman and Genevieve Masefield sail from Liverpool on the maiden voyage of the Mauretania. While posing as a passenger George is in fact an undercover detective hired by the Cunard Line. Dillman and Genevieve endure a nightmare voyage during which severe weather batters the vessel relentlessly and keeps the passengers away from the decks. Dillman is instrumental in rescuing a crew member from being washed overboard but he is too late to save one of the First Class passengers from the same fate.

At first, it looks like a case of death by misadventure. But Dillman and Genevieve come to realise that it was an act of calculated murder, connected with the presence on board of a record shipment of gold bullion - twelve tons in all - sent from the Bank of England.