Stumbling upon the unknown is always such a satisfaction. With upcoming movies such as Agent ZigZag and ITV's Foyle's War, here's a BBC series I have found called Cambridge Spies that tell's the other side of the story. It's probably old news to everyone across the pond but this is the first I've heard of it over here. Starring Tom Hollander, Toby Stephens, Rupert Penry-Jones, and Samual West.
"This is the story of the most notorious double agents in the history of spying. Four very British traitors". - Episode 1 opening
The Cambridge Five
Based on "The Cambridge Five" (also known as Cambridge Four, probably quite a few more members...), a group of spies who leaked information to the Soviet Union during World War II. Kim Philby (cryptonym: Stanley), Donald Duart Maclean (cryptonym: Homer), Guy Burgess (cryptonym: Hicks), Anthony Blunt (cryptonym: Johnson) composed the main four individuals. The fifth member has been identified by Oleg Gordievsky as John Cairncross (cryptonym: Liszt). Many of them were strategically positioned, Philby was a British Intelligence officer in Washington D.C., Burgess was the secretary of the British Foreign Minister of State, Maclean was the Secretary at the British Embassy of the Combined Policy Committee on Atomic Development, and Blunt joined MI5, the British Secret Service.