In neutral Turkey in 1944, German ambassador Franz von Papen and his British counterpart Sir Frederic Taylor attend a reception. Von Papen encounters Countess Anna Staviska, who is a Frenchwoman and the widow of a pro-German Polish count. Now destitute, the countess volunteers to become a spy for a fee, but she is turned down. A man approaches a German embassy attaché, Moyzisch, offering to provide von Papen with top-secret British documents for a price; £20,000 British. What is not yet known by the Germans is that the man, Diello, is the personal valet to Sir Frederic as well as the former valet of the late count. The documents taken from Sir Frederic's safe and photographed prove genuine. Diello is given the code name "Cicero" and asked to continue his subterfuge. Diello gives his money to Anna for safekeeping and pays her a portion of it, provided he be allowed to use her new villa as a meeting place for his transactions. When the valet also tells Anna his dream of living in South America together, she slaps his face but she agrees to his conditions. Moyzisch is summoned to Berlin by SS General Kaltenbrunner suspicious of Cicero's true intent. Allied bombing of a Romanian oil refinery is carried out, exactly as Cicero's photographed documents had outlined. Colonel von Richter is sent to Ankara to take over the negotiations with Cicero, while the British send a counter-intelligence man, Colin Travers, to identify the spy. Anna's newly-found wealth and previous willingness to become a spy cause her to fall under suspicion from Travers, who also rigs the ambassador's safe with a burglar alarm. Von Richter requests a document detailing an Allied operation called "Overlord". It is the D-Day invasion plan and Cicero wants £40,000 for it. Diello realises that he could soon be killed by one side or captured by the other. He decides to leave for South America, only to discover that Anna has stolen all of his money and departed to Switzerland instead. A letter arrives from her to Sir Frederic that identifies his valet as the spy being paid by the Germans. Diello breaks into the safe and photographs the D-Day plans and intercepts the letter but when returning the plans to the safe, he sets off the alarm and must flee. He now knows for certain how Anna feels toward him. Broke and on the run, Diello demands a £100,000 payment from the Germans for the photographs of the D-Day plans. He receives it, then manages to avoid getting killed or captured. A second malicious letter from Anna to the Germans misinforms them that the valet is a British spy, whereupon they dispose of the D-Day information as unreliable. Diello escapes alone to Rio, where he is shown enjoying a new life of prosperity and freedom. Or at least he is until Brazilian authorities arrive to take him into custody because all of his money is counterfeit, from Operation Bernhard. Anna's money in Switzerland, he finds out, is fake as well, which offers him consolation in his new, dark circumstances.
