Paris, 1 August 1914. 22-year-old Lena Schmidt is woken by her father Karl to say that war has broken out between France and Germany. Their shop is full of hostile customers who accuse Schmidt of being a German spy. He calls the police, but they arrest him and Lena instead.
Lena and her father are sent to separate internment camps. The vehemence of her protestations, both in person and in intercepted letters, suggest to a bald-headed intelligence officer that she might have potential as a spy.
Lena escapes the camp after seducing a guard and stealing his gun, tossing it into a puddle after leaving. She reaches a town, and sits in the railway station waiting room. A woman takes pity on her and offers food. Alarmed by the presence of uniformed men, Lena takes the next train to Dijon.
On the train, she is startled to see the bald intelligence officer in the corridor. He assures her that she is not going back to the camp, because she has two useful assets: fluent German and no relations. This is the first that Lena has heard about her father dying in the camp.
Berne, Switzerland. Now a fully trained spy working for the French Secret Service, Lena seduces her German opposite number Franz Abel, gets him drunk and secures top secret information. Back in Germany, Abel is found guilty of high treason, and shot.
Determined to take revenge, the German government asks for Lena to be deported there - a legal request, since she is a German citizen. To prevent this happening, she is ordered to marry a French officer. Volunteers are asked to draw cards. The holder of the ace of hearts, Lieutenant Pierre de Montmailon, agrees to marry Lena, on one condition.
Pierre and Lena are married. They move into a furnished hotel suite, and Pierre is disgusted by the ornate decorations, as he favours simplicity in design. He wrenches them off with a poker and smashes them on the floor. Lena realises that she is falling in love.
Lena and Pierre return home from an evening out to find the bald intelligence officer there. He insists on speaking to Lena alone, and says that she has to leave for Spain. He brushes aside Lena's protestations, saying that her marriage is one of convenience. After he leaves, Lena tearfully explains to Pierre that she has to obey orders, but they'll be together for good after the war.
The German Secret Service briefs one of their best agents, J14, telling him to pass on fake information to her, and to let her see through whatever role he adopts.
In Barcelona, Lena is disguised as a successful cabaret singer. In her dressing room, she is told that Don José Ramio, an immensely wealthy playboy, wishes to see her. It is J14.
November 1918. Armistice is declared, and Pierre and Lena start planning their belated honeymoon. A message arrives from the Ministry of War confirming the impending annulment of the marriage. Pierre confesses to Lena that this was his condition of going through with it, but he's since changed his mind.
Lena is told that she will be of more use to the French Secret Service unmarried. She protests that the war is over, and is told that this is irrelevant: she remains a French agent. She tells Pierre that there's nothing she can do.
Lena returns to her father's shop, and is greeted warmly by its new owner. She visits her old room and lies on the bed, where she is assailed by disturbing memories. Returning downstairs, she is abused by the shop's customers, but passionately defended by the new owner. Outside, the bald intelligence officer meets her and takes her to headquarters.
Lena is told that the information passed to her by J14 led to the sinking of three French transport ships, and that the German Secret Service has supplied them with evidence that she was working for them. A horrified Lena is told that she will be put on trial for high treason, leading to almost certain execution. She is taken into a van and driven off. Pierre runs after her, but is splashed with mud.
