As World War I rages in continental Europe, two New York heiresses, Sydney and Brooke Sinclair, are due to set sail for England. Brooke is engaged to marry impoverished aristocrat Edward Thorpe-Tracey, the future Lord Northbrook, in the wedding of the social calendar. Sydney has other adventures in mind; she is drawn to the burgeoning suffragette movement, which is a constant source of embarrassment to her proper sister. 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. In London, Isabel Nelson 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.