“Our world is in peril,” a narrator declares at the beginning of “Remember This: The Lesson of Jan Karski.” “Millions are being displaced, driven from their homes, impoverished, denied justice simply because of who they are, sickened, silenced, forgotten.”

He adds, after a pause, “We see this, don’t we? How can we not see this?” Moments later, he answers this rhetorical question: “Human beings have infinite capacity to ignore things that are not convenient.” 

It was ever thus, of course, and at least part of the mission of “Remember This” — a one-act piece co-written by the artistic and executive director of the Laboratory for Global Performance and Politics at Georgetown University,  Derek Goldman, and one of his former students, Clark Young, and starring the veteran stage and screen actor David Strathairn — is to put the horrors documented in today’s news reports into their proper historical context. That means taking a good, hard look at the bloody century that preceded this one, and at one mind-boggling atrocity in particular.

There are reasons the Holocaust still occupies a unique niche in the annals of ignominy. While Stalin and Mao were technically responsible for more deaths than Hitler, the systematic dehumanization and extermination of 6 million Jews — the culmination of nearly two millennia of ostracism and oppression — remains notable not only for its scale but for the egregious efficiency with which it was carried out as, indeed, the world largely refused to bear witness.

One man who did not look away was Karski, a Catholic Pole who was a hero of his country’s resistance and tried to serve the Jewish people specifically in that capacity. Trained as a diplomat, Karski became a courier for the Polish Underground during World War II, using his sharp memory to retain and report details of the German occupation. In 1942, after enduring torture and narrowly escaping death, he accepted an invitation to visit the Warsaw Ghetto and a Nazi concentration camp.

What Karski saw there defied all the notions of righteousness and decency instilled in him by his pious mother. “My faith tells me the second original sin has been committed by humanity,” he says in the play. That message was reinforced when Karski made appeals to President Roosevelt and the British foreign secretary, Anthony Eden, to try to stop the genocide, to no avail.

Karski only shared these experiences publicly decades later, when as a professor at Georgetown’s School of Foreign Service he agreed to be interviewed by the French filmmaker Claude Lanzmann for a landmark documentary, “Shoah.” A short clip from that film is shown early in “Remember This,” after which Mr. Strathairn — the narrator — morphs into Karski, donning a smart jacket and tie draped over a chair in Misha Kachman’s bare-bones set. This older gentleman is always “immaculately dressed,” he tells the audience, adopting a Polish accent.

It’s a rare moment of levity in the production — now making its New York premiere at Brooklyn’s Theatre for a New Audience — in which Mr. Strathairn, directed by Mr. Goldman, relays a first-person account of Karski’s harrowing journey during the war. The text is simple, almost to a fault, reflecting the humility and shrewdness that led this paragon of moral courage to define himself as “an insignificant little man,” a line used three times over the course of 90 minutes. 

Still fit and trim at 73, Mr. Strathairn evinces Karski’s physical bravery, simulating his leaps from a moving train and the window of a hospital where Gestapo thugs are looking to collect him for interrogation. Roc Lee’s original composition and sound design provide dramatic embellishment when the described action invites it, and more subtle, mournful accompaniment at other points.

Of course, the depravity and despair documented in “Remember This” require no effects to enhance their sting, from the “babies with crazy eyes” Karski witnesses in Warsaw, trying in vain to suck nourishment from their starving mothers, to the “collective moving body” of doomed men, women, and children crowded into the death camp he later visits.

The final moments of “Remember This” find Mr. Strathairn transitioning back into the role of narrator, once again pondering humanity’s capacity for crimes against itself. The play reminds us that, unfortunately, such reflection remains necessary, and it does so with a directness, urgency, and grace worthy of Karski’s legacy.