In 1915 Berlin, the German high command grows anxious about Turkey’s alliance as British attacks on the Dardanelles hint at inside knowledge. Von Sturm, the head of German intelligence, suspects Ali Bey, the Turkish commander in the region, as the traitor responsible.
When his best agent has not been heard from in weeks, Kruger is given the assignment. Shortly afterward, Annemarie, known by the code name “Fräulein Doktor,” returns after completing her mission. She warns Von Sturm that Mata Hari has fallen in love with her target and can no longer be trusted. She also recommends sending an incriminating message using a code she believes Mata Hari has passed to the Allies. She further reveals that Kruger is not who he seems, uncovering that he is actually British double agent Bertram Church.
When Kruger is arrested at his dentist contact’s office, another patient, Douglas Beall, an American studying medicine in Germany, is taken into custody as well. Beall is later released. To be thorough, Von Sturm orders Annemarie to confirm Beall’s innocence.
Annemarie arranges a rescue from an unwanted “suitor” by Beall, who invites her to his hotel suite. During the evening, he confesses he has fallen in love with her, who is now going by the name Helena Bohlen. Helena is drawn to him, but when she reads a coded message from Von Sturm informing her that she has followed Mata Hari’s supposed guidance, she abruptly leaves.
Beall persists, however. When Helena boards the train to Constantinople, he follows on impulse and continues courting her, despite her half‑hearted attempts to discourage him. Her assistant Karl watches with growing concern. As they near the Turkish border, she orders Karl to return to Germany so Beall can use his visa.
To answer Beall’s persistent questions, Helena must admit she is a German spy. This revelation does not diminish his love. Meanwhile, she captivates Ali Bey with her beauty. When she accepts his invitation to dinner, she poses as British agent K-6 and negotiates for vital information. To gain his trust, she arranges for Beall to be framed as a spy by Von Sturm (who has arrived after becoming concerned by Karl’s reports) and supposedly executed by firing squad, though a French prisoner is to be substituted. His suspicions (and jealousy) are allayed, and Ali Bey is arrested by his Turkish superiors.
When Von Sturm finally confesses that he could not complete the switch and that Beall really was shot, Helena loses her sanity and is confined to a nunnery. Refusing to accept the truth, she remains convinced that her lover will find her and take her away. It turns out Von Sturm had lied in an attempt to avoid antagonizing a then-neutral America. In the finale, Helena is reunited with Beall.
