One of the more insidious aspects of the Nazi regime is the way it forced so many into complicity, condemning them to a limbo between innocence and guilt. László Nemes’ film “Son of Saul” takes us into the lives of the Sonderkommando, the Jewish prisoners who ushered their brethren into the showers, disposed of the corpses and kept the death factories running smoothly. The Sonderkommandos were treated better than others, but their lives were short, too. The Nazis usually killed and replaced them every few months.

“Son of Saul” opens with Saul Auslander, a Hungarian Jew, doing his job on a typical day at Auschwitz-Birkenau. It’s a stunning sequence, a whirlwind of chaos and carnage compressed into a swift and efficient process, all filmed in one continuous, chilling shot. What drives home the horror is the face of Saul, played by Géza Röhrig (a Hungarian poet and sometime actor based in New York). Saul looks utterly blank — if anything, bored and slightly irritated — even when his young son dies before his eyes.

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Here, “Son of Saul” presents a mystery. It seems impossible that even the most dehumanized person could watch such a thing so emotionlessly. What’s more, Saul’s fellow prisoner, Abraham (Levente Molnár), insists that Saul has no son. Abraham is laying plans for a revolt, but Saul is uninterested. Saul will help only if it furthers his one goal: to find a rabbi and give his son a proper Jewish burial.

Inspired by what are known as the Scrolls of Auschwitz — testimonials from Sonderkommandos discovered buried in the camp’s crematoria — “Son of Saul” is an authentically detailed and harrowing film. It is also a frustrating one. Does this dead boy represent Saul’s last shred of humanity? Is it an albatross of guilt? Or is it a false and debilitating memory that prevents Saul from moving forward? Abraham makes a case for the latter when he tells Saul, “You failed the living for the dead.”

That’s a strange and dispiriting message, especially from a film that otherwise vividly reminds us why such memories should never be forgotten.