This WWII espionage history reveals how a British counterintelligence program turned Nazi spies into valuable double agents.

Far from the battlefields of the Second World War, a secret conflict of intelligence and counterintelligence was being waged. As German spies infiltrated the United Kingdom, they were captured by MI5—and offered a deal. Through the Double Cross System, they could turn on their own country and spy for the British.

The Double Cross System and the spies it produced saved thousands of Allied lives. They even contributed to the success of the D-Day landings at Normandy. Double agents helped convince Nazi Germany that the Allied invasion of Europe would take place across the English Channel, at Calais. One double agent was so good at what he did that Germany awarded him the Iron Cross, whilst Britain made him a Member of the British Empire (MBE).