A British spy pretends to be a Nazi officer and with help from the underground sabotages a poison gas factory in Czechoslovakia.
In 1940 Captain Stevenson (Robert Donat) defuses a 2,000 pound bomb in a London hospital. Gen. Weymouth sends him on a spy mission to his homeland of Romania. He packs and says goodbye to his mother. Stevenson parachutes and gets to Bucharest to meet his contact. He uses the alias Tartu and in Hungary is welcomed by Germans. Tartu rents a kitchen with a back door in Czechoslovakia and meets Maruschuka (Valerie Hobson). He is given a Nazi uniform, and in a bomb factory a woman is arrested for sabotage. At home Tartu protects Paula Palacek (Glynis Johns) after she killed a German officer. He asks her to help him reach the underground. Tartu takes aristocratic Maruschuka out to dinner, and she says she will help him find the underground. She kisses Tartu, and inspector Otto Vogel (Walter Rilla) sees them. At home Maruschuka consoles crying Paula.
In the factory Paula is caught for sabotage. Tartu has to turn her in but does so privately. Paula is shot. Maruschuka goes to Dr. Novotny (Martin Miller) and reports to the council about Tartu. They tell her that Tartu turned in Paula. Novotny tells her to use her influence on Tartu but not to kill him directly because Germans threaten to kill a hundred for each one of theirs killed. Maruschuka tells Otto that she thinks Tartu is a spy and urges him to kill him. Tartu gives her a bracelet, and she lets Otto hear their conversation; but officers begin checking papers and shoot a man who runs. Maruschuka and Tartu make another date.
Tartu goes to the gas plant through a metal detector and works with chemicals. At home he learns that Maruschuka went out with Otto. Tartu drinks and makes fun of Czechs. He staggers and is carried off by men. Tartu asks for help in destroying the gas plant and gives his British name. He is knocked down, and they say they are Gestapo and let him write a letter. Novotny comes in and congratulates Tartu on finding the underground. They help him prepare compact explosives. Tartu gets into the plant. Otto tells Maruschuka that he is checking on Tartu and learns that the real Tartu is dead. She realizes her mistake. When Otto calls the gas factory, she knocks him dead. Maruschuka goes to the gas plant to warn Tartu, who tells her to have Novotny bring a car. Tartu places the explosives while he is hunted. He grabs a gun from a guard and runs out. Tartu gets in the car and sees the explosion. In the final scene Tartu leaves with Maruschuka in a German plane they steal.
Based on an actual incident, the horrifying Nazi occupation
makes this spy thriller very realistic with many twists and turns
in the desperate circumstances.