Benjamin B. Ferencz is now 98, but in “Prosecuting Evil: The Extraordinary World of Ben Ferencz,” there’s no sign that his passion has dimmed. In this documentary from the Canadian director Barry Avrich, he tells of how he emigrated to the United States from the tumult of Eastern Europe in the 1920s as a child and grew to become the chief prosecutor in the Einsatzgruppen case at the Nuremberg trials.

Ferencz was 27 when the case began and apparently not tall enough for the lectern without elevation. In the film’s best stretches, he offers a detailed account of the proceedings and what followed, including his encounter with the convicted war criminal Otto Ohlendorf shortly before Ohlendorf’s execution.

He also takes us through his post-Nuremberg work, such as how he used a set of bones from Auschwitz to persuade the German government to maintain the country’s Jewish cemeteries in perpetuity and his advocacy for a permanent international criminal court. He explains how he and the Vietnam-era defense secretary Robert S. McNamara came to write an odd-couple Opinion article in The New York Times in 2000 urging Bill Clinton to sign the treaty creating that court, even though McNamara could have been one of the first defendants.