Screenshot depicting London during the Blitz in 1940
British Captain Terence Stevenson (Robert Donat) accepts an assignment even more dangerous than his everyday wartime job of defusing unexploded bombs. Fluent in Romanian and German and having studied chemical engineering, he is parachuted into Romania to assume the identity of Captain Jan Tartu, a member of the fascist Iron Guard. He makes his way to Czechoslovakia to steal the formula of a new Nazi poison gas and sabotage the chemical plant where it is being manufactured.
However, his contact is arrested before he can arrange for a job in the factory. Tartu is instead assigned work as a foreman at a munitions factory, where he is issued a German uniform. He is billeted in the house of Anna Palacek (Phyllis Morris) and her daughter Pavla (Glynis Johns), who works in the plant; also living there are German Inspector Otto Vogel (Walter Rilla) and the lovely Maruschuka Lanova (Valerie Hobson), who lives well by making herself popular with the German officers.
That day Pavla shoots a German who earlier had had the man she loved executed. Tartu provides her with an alibi, winning her trust, and then reveals himself as a secret agent. Needing to get into the chemical plant, he asks her help to contact the Czech underground, and is surprised when she contrives for him and Maruschuka to go on a date. They talk guardedly but make it clear they are both working against the Nazis.
At work the next day, Pavla is seen attempting sabotage. She whispers to Tartu to protect himself by denouncing her. He does, and she is summarily executed. The factory manager rewards Tartu by arranging the transfer he wants, to the chemical plant.
The same day, Maruschuka contacts Dr. Novotny (Martin Miller), the leader of the local resistance group, and says she trusts Tartu. However they have already concluded that he had saved Pavla as a trick to win the confidence of the underground and order him killed. To arrange this while avoiding the execution of 200 Czechs by the Nazis in retribution, Maruschuka returns home and comes on seductively to Vogel. She tells him she's sure Tartu is a spy and Vogel says he will call the Gestapo. But she suggests Vogel advance his career by taking the initiative and killing Tartu himself. Vogel wants evidence, so she goes out again with Tartu that night, so that Vogel can overhear them talking. But they are interrupted before this can happen.
The next day, Tartu goes to work for Dr. Willendorf (Percy Walsh), the head of the chemical plant. He is dismayed to learn that the first shipment of gas is scheduled for the following night. Desperate to reach the resistance, he pretends to get drunk in a bar and brags that he knows of six Czech resistance members about to be arrested and killed. His idea works: he is abducted by the underground, and with a great deal of effort, finally convinces them they are on the same side. Working all night, they manufacture enough miniature bombs to demolish the plant if properly placed.




Screenshot of the climactic escape
At the Palacek house, Vogel checks up on Tartu's credentials and happily informs Maruschuka that Tartu is indeed a spy. She believes Vogel and, trying to stop him telling anyone else, accidentally kills him. She hurries to the chemical plant to warn Tartu that he is in danger. He sounds an air raid alarm and runs through the plant setting the bombs in their places. The Germans quickly realize he is a saboteur, but he just manages to complete his task and escape from the heavily guarded plant, which blows up as he is driven away.
Finally he, Maruschuka, and a pilot steal a German bomber and fly to safety as he proposes "just a simple little wedding".
Who goes out again with Tartu?
Maruschuka