In the Summer of 1940, after the Nazi occupation of Poland, an underground operative called Witold Pilecki accepted a mission to uncover the fate of thousands of people being interred at a new concentration camp on the border of the Reich. His mission was to report on Nazi crimes and raise a secret army to stage an uprising. The name of the detention centre -- Auschwitz. Over the next two and half years, Witold forged an underground army that smuggled evidence of Nazi atrocities to the West, culminating in the mass murder of over a million Jews. His reports from the camp were to shape the Allies response to the Holocaust - yet his story was all but forgotten for decades.