What did it take to lure the two of us back to our old stomping ground? A cracking expose on Australia's greatest untold spy story.It was only six years ago in 2017 that ASIO's Official History confirmed our spy agency had been penetrated by the KGB during the Cold War. But just what happened and who betrayed Australia has been a closely guarded secret. Until now.In this week's Four Corners, we reveal not just the full extent of this breathtaking treachery but also the identity of the ASIO officer who became a KGB mole.At the time, he was a senior manager with top secret clearance who sold out his colleagues and his country for cash payments from the Russians. He was considered one of the KGB's most valuable western intelligence assets during the Cold War.For the first time, former colleagues also speak about the shock and trauma of realising they had a traitor inside their office.Because of his access to critical secrets shared among the Five Eyes intelligence partnership, the mole's actions also jeopardised Australia's intelligence alliance with the United States and the United Kingdom.It's a genuine spy thriller with hidden compartments in diplomatic cars, exercise groups acting as diversions at Sydney's Bondi Beach and so-called 'dead drops' of classified documents at night in Centennial Park.