Adapted from a true story, a greedy valet in a British embassy sells top-secret documents to Germans in 1944 with assistance by an impoverished countess.
In Ankara the Countess Anna Staviska (Danielle Darrieux) asks Count Franz von Papen (John Wengraf) for money and offers to help Germany. Ulysses Diello (James Mason) tells L. C. Moyzisch (Oskar Karlweis) he will sell him a film of top-secret documents for £20,000. British ambassador Frederic (Walter Hampden) tells his valet Diello that Anna lost her wealth. Papen gives Moyzisch the money.
Diello gives Moyzisch the film which he develops while Diello takes the money from his safe. Diello calls on Anna and gives her £5,000. He asks her to deposit money for him and provide a place for business. He was her husband’s valet and says she is attracted to him. She slaps him but agrees on business.
Col. Von Richter (Herbert Berghof) tells Moyzisch they will verify an attack in Romania. Papen learns it occurred without a warning and asks to see Anna. Diello opens the British safe and photographs documents. The British learn that Papen has secret information.
Colin Travers (Michael Rennie) comes to Ankara and questions Frederic. Travers says that Anna is entertaining. Richter gives orders to Papen and Moyzisch. Anna takes Richter to Diello, who is glad to deal with him. Diello and Anna kiss.
Diello has sold secrets for £150,000, but the Germans are afraid to use them. Travers asks Diello about Anna. Travers tells Frederic they learned the German code. Diello tells Richter that Anna is suspected by both sides. Richter asks Diello about Overlord, the invasion of Europe. Travers and Frederic learn that Papen says a spy is in the British embassy. Keith MacFadden (Roger Plowden) puts an alarm on the safe.
Anna shows Diello their passports and has arranged transportation to Brazil. Travers tells Diello that Anna flew to Switzerland with £130,000. Diello goes out and calls for her, the bank, and Richter. Diello knows he is watched and plans to meet Moyzisch in Istanbul. Diello notices an alarm, breaks the circuit, and opens the safe. He photographs papers but is interrupted by a cleaning lady, who connects the circuit to vacuum. When Diello closes the safe, the alarm sounds. He runs out, and Travers sends Johnson after him. Travers tells MacFadden they must kill Diello.
Travers sees Diello board a train, but they are not allowed to search. Diello reads the letter from Anna that accuses him. Diello gives film to two Germans and asks for £100,000. Travers follows Moyzisch, who gives the money to Diello. Travers sends a note to Diello that they will protect him. Diello goes with Travers and MacFadden, and two Germans follow. Diello runs off. Moyzisch tells Richter that D-Day will be in Normandy. Richter gets a letter that Diello is a British agent and throws away the information.
Diello in Rio de Janeiro is told his money was counterfeited in Germany and has also turned up in Turkey and Switzerland. Diello laughs because Anna is caught too.
This spy thriller depicts a clever British servant who is tired of being treated as an inferior and tries to secure happiness with money and a beautiful woman while betraying the Allied nations, but ironically she and he are outsmarted by the Germans who in turn outsmart themselves by not believing the information.