Arrested as a collaborator after the war, Rabbi Murmelstein was cleared after an investigation failed to support the charge. (He died in 1989.) For many survivors, the exoneration did not significantly rehabilitate his image. Gershom Scholem, the famed historian and philosopher of Jewish mysticism, denounced Rabbi Murmelstein, his erstwhile friend and publisher, publicly calling for him to be hanged.

Stephen D. Smith, executive director of the University of Southern California Shoah Foundation and the chairman of genocide education for Unesco, points to the ethical quicksand in which the rabbi found himself. “So many Jews were caught up in this gray zone of moral ambivalence,” Dr. Smith said. “Those are mitigating circumstances that need to be understood. Because it won’t be the last time human beings are put in this situation.”

Rabbi Murmelstein lived out his life in self-imposed exile in Rome, where Mr. Lanzmann tracked him down in 1975 and shot nearly a dozen hours of conversation. At times the rabbi is combative, but the two are well matched: Mr. Lanzmann is an intellectual who spent his youthful years in the wartime French Resistance and later forged intimate relationships with Jean-Paul Sartre and Simone de Beauvoir. We watch him sparring, cajoling, arguing, digging ever deeper into the recesses of the rabbi’s memory and self-justification.

Yes, Rabbi Murmelstein told him, Jews died on his watch. But under his leadership, typhus was eradicated, conditions for the sick improved, and, he argued, the ghetto itself was protected by the very “embellishment” fashioned for visitors that patched over the horrors occurring within its walls.

“In Theresienstadt, those who weren’t indispensable were in danger,” Rabbi Murmelstein, jowly and avuncular in tweeds and tortoiseshell glasses, tells Mr. Lanzmann. “I had a bad reputation. They said I was a bigmouth and mean. But the old people were taken care of. They no longer slept on the floor but in beds with clean sheets, cared for by nurses.”

By the end of the week, filmmaker and subject are joking like old friends; a bond has been forged. “I’d compare myself to Sancho Panza,” the pugnacious Rabbi Murmelstein tells Mr. Lanzmann. “He’s pragmatic and calculating, while others are tilting at windmills.”