Actor Michael Raver has delivered some of the best performances seen on stage in the first five seasons of the Sarasota Jewish Theatre, but they were just preludes to the commitment and diversity of characters he displays in the powerful one-person play “Remember This: The Lesson of Jan Karski.”

The play by Clark Young and Derek Goldman, beautifully and sensitively directed by Gus Kaikkonen, launches the company’s sixth season.

It is the true story of a Polish man who became an unintentional international hero, risking his life to reveal the atrocities he witnessed in Jewish ghettos and concentration camps to a world that didn’t want to hear and too often didn’t believe him.

Raver plays nearly three dozen characters, from his loving and encouraging mother, to his dancer wife, neighborhood friends and world leaders.

But the focus is on Karski, a Catholic who was proud of his Polish heritage and homeland. As Nazis took over Poland, he got swept up in the resistance movement, observing, reporting and trying to avoid savage treatment.

Despite his heroic efforts, Karski repeatedly describes himself as an “insignificant little man,” repeating the words in pointed ways when he is stunned by the essential rejection of his stories by British leaders, Supreme Court Justice Felix Frankfurter and eventually President Franklin Roosevelt.

“Human beings have an infinite capacity to ignore things that are not convenient,” he says at the play’s start, while asking the audience, “What can we do?” and  “Do we have a duty or responsibility as individuals to do something, anything?”

On a simple set with a couple of wooden chairs and a large desk, Raver shares Karski’s story of being held captive by the Germans, escaping from a train filled with Poles bound for a labor camp and being secreted into a Jewish ghetto and a concentration camp so that he can later report what he saw to world leaders as an eye-witness.

The torture scenes are particularly hard to watch, as Raver throws himself around the stage, making you feel each jab, punch or strike, and making it seem like you can see his captors at work. He effortlessly switches characters with a shift in posture, a break in his voice or a change of accent, as he depicts Germans, Poles, Hungarians, Brits and Americans with ease.

As Frankfurter, who refers to Karski as “young man,” he holds an unseen cigarette between two pinched fingers, blowing invisible smoke into his visitor’s face. His Roosevelt conveys the familiar cadence of the president’s voice, as he smokes through his long cigarette holder.

There is a beautiful joy as we see the young man riding his bicycle through the beautiful Polish countryside and hearing his mother tell him to climb the ladder to whatever opportunities he chooses to pursue. And then we feel the devastation and disgust as he describes naked bodies piled in a ghetto ditch or the cackle of German soldiers talking about the pain the Jews will suffer as they are exterminated.

Videos and photos of the real Karski are displayed at times on a screen, along with images of what he is describing. He talks about his regret in participating in director Claude Lanzmann’s epic nine hour documentary film “Shoah,” which was released in 1985. By then Karski was well into his long tenure as a professor of international relations and Polish history at Georgetown University.

As difficult as some of parts of the story are to watch and hear, the questions Karski asks at the beginning are the most troubling aspect. Why were leaders complacent when faced with details about the Holocaust? Why did they not act more quickly? And what can we, as individuals, do in the future to avoid such atrocities?

‘Remember This: The Lesson of Jan Karski’ by Clark Young and Derek Goldman. Directed by Gus Kaikkonen. Presented through Feb. 15 by Sarasota Jewish Theatre, at the Sarasota Players, 3501 S. Tamiami Trail, Sarasota, Suite 1130. Reviewed Feb. 4. Tickets are $38-$50, $19 for students. sarasotajewishtheatre.org; 941-365-2494.