As the First World War rages in continental Europe, two New York heiresses, Sydney and Brooke Sinclair, are due to set sail for England. As international tempers flare, the German embassy releases a warning that any ships making the Atlantic crossing are at risk. Undaunted, Sydney and Brooke board the Lusitania for the seven-day voyage with Edward, not knowing that disaster lies ahead. In London, Isabel Nelson, a young woman grateful to have escaped her blemished reputation in Oxford, has found employment at the British Admiralty in the mysterious Room 40. While she begins as a secretary, it isn't long before her skills in codes and cyphers are called on, and she learns a devastating truth and the true cost of war. As the days of the voyage pass, these four lives collide in a struggle for survival as the Lusitania meets its deadly fate.