In this two part special, best-selling author Ben Macintyre explores Kim Philby, the most famous double agent in history. The key to Philby's success, and his survival, lay in his friendship with a man that history has been largely forgotten - Nicholas Elliott. Elliott was Philby's colleague in MI6 who befriended him, defended him, and unwittingly supplied him with secrets until he discovered the truth. Philby was a man of contradictions - charming and courteous, but also a fanatic and a ruthless killer who deceived his colleagues not only in MI6 and MI5, but also in the CIA, most notably James Jesus Angleton. Filmed in locations from London to Istanbul, Washington DC to Beirut and finally in Moscow, it features chilling first hand testimony from those who knew Philby and is supported by archive footage, and never before seen photographs.