The Mysterious Lady (1928)

Austrian Captain Karl von Raden attends the opera one evening, and meets Tania. After the performance, he takes her home, and the two of them spend the next day on a romantic outing. That evening, Karl must deliver some important plans to Berlin. Just before boarding his train, he learns that Tania is really a Russian spy. She comes to see him aboard the train, and admits that she set things up on purpose so as to meet him, but she also insists that she truly has fallen in love with him. When Karl rebuffs her coldly, she steals the plans, which leads to him being court-martialed and imprisoned. Karl's influential uncle is able to provide him with one last chance to clear his name.