Hubert Cornfield - Operation Cicero (1956)

(from tv.com) :
Based on the true story of a World War II spy with access to top secret information in the British embassy in Turkey, this episode follows the brief but bright career of an English valet, code-named Cicero, who seized his chance to sell expensive secrets to the Germans, including details about D-Day’s Operation Overlord. The Nazi government, the British ambassador, and a Polish countess all dance on strings even as British counteragents close in.