Spanish photographer Francesc Boix, imprisoned in the Mauthausen-Gusen concentration camp, works in the SS Photographic Service. Between 1943 and 1945, he hides, with the help of other prisoners, thousands of negatives, with the purpose of showing the freed world the atrocities committed by the Nazis, exhaustively documented. He will be a key witness during the Nuremberg Trials.

Directed by Mar Targarona