During World War II, Kurt Gerstein (Ulrich Tukur), a Waffen-SS officer employed in the SS Hygiene Institute, designs programs for the purification of water and the destruction of vermin. He is shocked to learn that the process he has developed to eradicate typhus, by using a hydrogen cyanide mixture called Zyklon B, is now being used for killing Jews and other "undesirables" in extermination camps. Gerstein attempts to notify Pope Pius XII (Marcel Iureş) about the gassings, but is appalled by the lack of response he gets from the Catholic hierarchy. The only person moved is Riccardo Fontana (Mathieu Kassovitz), a young Jesuit priest. Fontana and Gerstein attempt to raise awareness about what is happening to the Jews in Europe but even after Fontana appealing to the pope himself, the Vatican makes only a timid and vague condemnation of Hitler and Nazi Germany.