In the spring of 1918, a German U-boat stops a Netherlands freighter on its way to Stockholm, Sweden during the final year of World War I. The Germans board the ship, identify one man as a Belgian spy, and take him prisoner. One of the passengers questioned is an attractive Switzerland dress shop owner named Madeleine Goddard (Vivien Leigh) who is returning to Stockholm from a business trip to Paris, where she purchased dresses for her boutique. After returning to Stockholm, Madeleine, who is a German spy, meets with her contacts and provides information on Allied troop movements she acquired in Paris. The information is cleverly concealed in the gowns she purchased. While the Germans believe that she is one of their top spies, Madeleine is in fact a French double agent working to uncover the identity of the new German secret service leader in Stockholm.