In October 1944, Jewish-Hungarian prisoner Saul Ausländer serves as a Sonderkommando in the harrowing gas chambers of Auschwitz. He endures the relentless horrors around him with a stoic demeanor, seemingly numbed by the daily atrocities. When Saul discovers a still-alive boy among the dead, who is being suffocated by a Nazi doctor that demands an autopsy, he is determined to give the child a proper Jewish burial. He approaches Miklós Nyiszli, a fellow Hungarian prisoner and a coerced assistant to the infamous Josef Mengele, pleading with him not to disfigure the boy's body. Although Miklós cannot grant Saul’s request, he allows him five minutes alone with the boy that night before the cremation.
Desperate, Saul seeks assistance from Rabbi Frankel, but is dismissed and encouraged to perform the burial ritual himself. As Saul copes with his sorrow, he overhears Sonderkommando Abraham talking about a potential uprising with Oberkapo Biederman. Biederman, initially wanting to document the camp's atrocities with a camera taken from the dead, believes this might attract attention and aid from the outside world. Saul offers to join their cause, feigning a repair on a lock while a fellow prisoner captures images of a cremation.
Determined to seek help, Saul sneaks into another Sonderkommando unit and finds a man known as the Renegade, whom he discovers at a riverbank where ashes are discarded. When the Renegade refuses to help, Saul discards his shovel into the water. After a brief struggle, both are captured and interrogated, leading to the execution of the Renegade while Saul is allowed to return.
Later, Saul searches Miklós's office for the boy's remains but is ridiculed by Nazi officers. Confronting Miklós afterward, he learns that the body is safe. He manages to retrieve the body and carries it back to his barrack in a sack. As tensions rise, SS-commandant Moll instructs Biederman to compile a list of seventy names, causing fear that his unit may face execution. In this tense environment, Abraham instructs Saul to procure gunpowder from a female prisoner named Ella. Amidst the grim reality, Ella recognizes Saul and attempts to reach out, but he withdraws, focusing on his grim task.
Saul searches for a rabbi among newly arrived Hungarian Jews, encountering a Frenchman named Braun, who misleads Saul into believing he is a rabbi. Under the covert guise of a Sonderkommando, Saul smuggles Braun into the camp. However, amidst the chaos in the woods, Saul loses the gunpowder package and falsely claims that the murdered boy is his illegitimate son to Abraham.
The abominable conditions escalate the next morning when prisoners are called back to work in the crematorium, only to discover that Biederman and his unit have been executed. Prompted by the dire situation, Abraham galvanizes the prisoners to rebel against the SS guards. In the ensuing turmoil, Saul seizes the boy's body and escapes into the woods with Braun and a few others, aiming to bury the boy by the river. It soon becomes evident that Braun is an impostor when he fails to recite the Kaddish prayer.
With the guards bearing down on them, Saul struggles to carry the body across the river but is thwarted and ultimately saved by Rabbi Frankel as the corpse drifts away. The remaining prisoners find refuge in a forest shed and discuss plans to join the Polish resistance. As they scheme, Saul catches sight of a young peasant boy peering into the shed. For the first time, he smiles, but the moment is cut short as the boy is seized by an SS officer before managing to flee into the woods, accompanied by the distant sound of gunfire.
