The story opens at an opera house where Captain Karl Von Raden finds himself being seated next to a woman expecting the arrival of her cousin who never comes. After the opera, Karl notices the attractive woman he sat next to now standing on the curb in the rain. She informs him that she has no money for taxi fair, so he agrees to escort her home himself. While at her apartment, they immediately embrace and have a romantic affair. Afterwards, Karl is told by an "Uncle Eric" that his encounter with a woman he met at the opera had been pre-arranged and that she is a spy. When the two meet again on a train, Tania tells Karl she loves him, but he refuses to believe anything she says. Knowing that Karl has in his possession valuable secret plans, she decides to carry out her orders and steal them, leaving him a note that they are now enemies. Now that it is known that he had an affair with a lady spy who has taken the secret plans, Karl is met with a court-martial, stripped of his military uniform and sent to prison. After a carefully planned escape, Karl heads for Vienna posing as a musician at a cafe, to avenge Tania and retrieve those secret plans.