As a young evangelical Christian initially supportive of Hitler's ascension, Kurt Gerstein becomes the only SS officer on record ever to alert the West and the Vatican about the extent of human horror transpiring in the camps. A fervent believer and Nazi Party member before the war, Gerstein grows disillusioned and bristles under the regime's repression of the Lutheran Church's activities. His outspokenness and decision to support a Jewish convert lands him in Nazi prison camps several times. After his release, Gerstein discovers that his mentally-ill sister-in-law, Bertha, has been euthanized at the notorious Hadamar Institute. Doctors try to disguise the larger scale extermination efforts taking place at Hadamar by blaming the many patients' deaths on an outbreak of typhus.
Dubious of the explanation and determined to ensure that what happened to his sister-in-law will never happen again, Gerstein joins the SS as a sanitation expert with an expertise in delousing methods. He quickly rises in the ranks but to his horror, sees that his delousing chemical of choice, Zyklon B, become the preferred murderous agent in the gas chambers. Gerstein decides to destroy shipments of Zyklon B and eventually tries to alert the incredulous West and the Vatican of what is transpiring.