In programme one of this two-part special, best-selling author Ben Macintyre explores the truth behind the most famous double agent in modern history - Harold Kim Philby.

He argues that Philby has become a caricature of the gentleman spy, obscuring the fact that Philby was a ruthless killer, who betrayed everyone around him.

The key to Philby's success and his survival, Macintyre explains, lay in his friendship with a man that history has 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.

This is the remarkable tale of two friends and two spies, each working on opposite sides in the Cold War.