As the First World War 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, not knowing that disaster lies ahead.