Audiences will not soon forget “Remember This,” a theatrical jolt on what it means to witness horror and be unable to stop it. The one-man play stars David Straithairn and was written by Clark Young and Derek Goldman; the latter is also the director. Produced by the Theater for a New Audience, it runs through the weekend at the Polonsky Shakespeare Center, at Brooklyn. 

“Remember This” originated at Georgetown University, where Jan Karski taught for four decades. Before he was lecturing by the Potomac, Karski, a native of Łódź, was a courier for the Polish resistance during World War II. He narrowly escaped the Katyn massacre, a blood-letting ordered by Stalin that saw 22,000 Polish military officers and intellectuals murdered.

Karski’s task was to convey the reality of the German occupation of his homeland to the Polish government in exile, headquartered at Paris. His dispatches also touched on the byzantine factionalism of the Polish resistance. Karski’s remit soon expanded to briefings of Allied grandees at both London and Washington, D.C. To these audiences, he brought ill tidings of Hitler’s war against the Jews.

Karski had a front row seat for the destruction of Polish Jewry. He was twice smuggled into the Warsaw Ghetto, and posed as a Ukrainian guard to gain admittance to a transit camp that served the Bełżec death camp, where nearly 500,000 Jews were gassed. Only seven survived. The impressions and microfilm he smuggled out documented the Holocaust as it was happening. 

For decades, Karski did not discuss what he had done to make Nazi crimes known. He did consent to an interview on Claude Lanzmann’s “Shoah,” though he was so perturbed by the Frenchman’s cuts that he wrote his own book. He was made an honorary citizen of Israel, and he is honored as a Righteous Among the Nations at Yad Vashem.

“Remember This” uses a minimalist set — a table, two chairs, and a pair of shoes — to tell the story of how Karski told his tale. Mr. Straitharn has clearly studied his subject and captures his dignity of bearing, implacable sense of mission, and ram rod Polish patriotism. The 90-minute show is an exercise in straightforward, earnest storytelling.

The most affecting moment of “Remember This” concerns Karski’s lament for the Bundist politician Szmul Zygielbojm, who sat on the National Council of the Polish government-in-exile. Zygielbojm did everything he could to goad the Allies into action. Karski’s guide in the ghetto was one of Zygielbojm’s Bundist comrades, and Karski and Zygielbojm met in London.

After the crushing of the uprising in the Warsaw Ghetto, Zygielbojm sent a note to the Polish president and prime minister. It reads, “I cannot continue to live and be silent while the remnants of Polish Jewry, whose representative I am, are being murdered. My comrades in the Warsaw ghetto fell with arms in their hands in the last heroic battle. I was not permitted to fall like them, together with them, but I belong with them, to their mass grave.”

Zygielbojm took his own life after his wife and son were killed in the Warsaw Ghetto’s final liquidation. He had given fair warning on the British Broadcasting Service, lamenting that it would be a “shame to go on living if steps are not taken to halt the greatest crime in human history.” Both Karski and Zygielbojm were carriers of knowledge that few were prepared to hear.

The Bundist hoped by his death by an overdose of Amobarbital would “save the living remnant of the Polish Jews from destruction.” Mr. Straithairn has Karski cry, “I am an insignificant, little man.” When it came to Zygielbojm, however, Karski issued this command: “Remember his name. This man loved his people more than he loved himself.”