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At the start of World War I, pretty Carla is befriended by German officer Rudolph Ritter while trying to buy a ticket to Vienna in a hectic Luxembourg train station. After the train dumps them at the Austrian border, Carla suddenly deserts her benefactor without explanation. In Vienna, Rudolph leads an effort to break a Russian spy ring, unaware that Carla, who is actually Russian, is passing and receiving messages under the code name K-14 while posing as a cabaret singer. After Carla is searched and almost caught with a message during a performance, she is reassigned to a seamstress' shop. When that cover is exposed, she flees Vienna one step ahead of Rudolph and becomes a nurse at a German military hospital. Later, one of Carla's messages, which she has written in invisible ink in a book, is intercepted by Rudolph as it is being passed from agent to agent along a convoluted route. After tracing the message's place of origin, Rudolph stations himself at the German post and vows that he will catch the spy. Soon after his arrival, Rudolph and Carla meet, and their romantic desires are rekindled. Against her better judgment, Carla falls in love with her enemy, and although he eventually uncovers evidence implicating her as the spy, Rudolph is reluctant to admit that she is K-14. Forced by his sense of duty, however, Rudolph traps Carla by passing her a phony order to meet a contact in a deserted cottage. Finally caught by Rudolph, Carla prepares to be arrested but is rescued by another spy, who wounds Rudolph during the escape. After the armistice, Rudolph and Carla meet again at a Swiss train station and, free at last from the obligations of war, fall into each other's arms.