For years during the Cold War the KGB's only access to US and UK secrets was through an Australian spy.

The ASIO mole handed over highly classified information for wads of cash, but none of his colleagues suspected a thing.

After the Soviet Union's collapse, information sourced through Russian KGB defectors sparked the most extensive mole hunt in ASIO's history.

Four Corners reveals the identity of the traitor, and the full extent of his betrayal.