There was something different about prisoner number 4859. Like many of his fellow prisoners, 4859 had been arrested by the Nazis and slung into a new camp on the outskirts of the town of Oświęcim. So, what set this particular prisoner apart from his fellow detainees? 4859’s real name was Witold Pilecki and, as The Volunteer, the new book by former Daily Telegraph war correspondent Jack Fairweather vividly documents, he was locked up in the infamous Auschwitz concentration camp voluntarily.

The story of Witold Pilecki was almost unknown in Poland before the fall of the communist regime in 1989. Indeed, to anyone who was aware of the name, Witold Pilecki was a traitor who had got what he deserved. The reality was very different.

Following the Nazi invasion of Poland in 1939, the occupiers quickly set about setting up a network of prison camps. Of those, Auschwitz would become the largest and most notorious in the Nazi concentration camp system.

After the camp became operational with the arrival of the first prisoners in May 1940, Auschwitz was something the Polish resistance was keen to know more about. What, exactly, was going on in the converted army barracks on the outskirts of the town of Oświęcim?

Enter Witold Pilecki. A seasoned and decorated veteran of the Polish-Soviet War of 1919-1921 who had seen action at the Battle of Warsaw, Pilecki was eager to help out the resistance in any way he could. He, too, was eager to find out what was going on in Auschwitz, and he had a plan how to do just that.

Pilecki’s plan was simple. He would get himself arrested, get thrown in Auschwitz and then start gathering and smuggling out intelligence about the place. He presented his plan to infiltrate the camp to his superiors in the Autumn of 1940, and they eagerly accepted.