Tinker Tailor Soldier Spy

About Tinker Tailor Soldier Spy: George Smiley, the aging master spy of the Cold War and once heir apparent to Control, is brought back out of retirement to flush out a top level mole within the Circus. Smiley must travel back through his life and murky workings of the Circus to unravel the net spun by his nemesis Karla 'The Sandman' of the KGB and reveal the identity of the mole before he disappears.

How do you find an enemy who is hidden right before your eyes?