An urgent new theatrical voice has arrived with “The Overwhelming,” J.T. Rogers’s etched-in-blood snapshot of Rwanda. This dense and devastating look at a well-meaning American family sliding into moral ambiguity isn’t always easy to watch, but even when an unwelcome strain of didacticism seeps into the narrative, Mr. Rogers and his masterful director, Max Stafford-Clark, make sure its lessons are hard to forget.

Jack Exley (Sam Robards), a languishing academic desperate to get back on the tenure track, thinks he has found his meal ticket in Dr. Joseph Gasana (Ron Cephas Jones), an old college roommate who returned to his native Rwanda to run a free HIV clinic. The timing couldn’t be worse: Jack, looking to turn Gasana’s story into a book, arrives from Milwaukee in early 1994, just before the roiling hatreds between the Hutu and Tutsi tribes exploded into genocide, and Gasana is nowhere to be found. To make matters worse, Jack has brought along his second wife, an essayist named Linda (Linda Powell), and his semi-estranged son, Geoffrey (Michael Stahl-David).

The family’s makeshift research trip almost immediately turns into a semester of living dangerously, complete with uncooperative government officials and ominous middle-of-the-night phone calls. Linda and Geoffrey each link up with Rwandans of murky allegiances, while Jack persists in searching for Gasana, whom nobody else seems terribly motivated to find. (The dynamic between Jack and Gasana is depicted primarily through letters the doctor wrote before Jack’s arrival, recited with haunting serenity by Mr. Jones.) All the while, the cresting tensions — 800,000 Rwandans, mostly Tutsis and neutral Hutus, would soon be slaughtered in the space of less than three months — threaten to reduce the fate of one Tutsi doctor to a drop in an ocean of blood.

In some ways, Jack’s efforts parallel those in the 1949 film “The Third Man,” with its wide-eyed American becoming immersed in the moral convolutions of a war-torn region as he looks for an old friend who may or may not be dead. And Graham Greene, that film’s screenwriter, would have recognized the sure-footed blend of potboiler and ethical treatise “The Overwhelming” provides.

Like Greene, Mr. Rogers has also opened himself to the criticism that people from other cultures become more interesting once white Americans drop down among them. (The decision to make Linda African-American feels devised to mute this criticism somewhat.) And the very idea of depicting the Exleys’ domestic woes alongside an entire nation’s implosion runs the risk of equating the two. But rather than paint the Exleys as beacons of enlightenment, “The Overwhelming” gives them an unearned strain of moral superiority. “You have to go beyond this,” Jack lectures his hosts on his second day in Rwanda.

Would an international relations professor, even an untenured one, stumble so naively into these thickets of tribal hatreds? Could a woman who says things such as “The dichotomy here is vertiginous” in conversation really find paying work as a writer? But these glitches notwithstanding, “The Overwhelming” stays true to the desperate, often ruthless calculations of any nation teetering on the brink of mayhem. “Stop thinking like an American,” Jack is counseled by Verbeek, an embittered nongovernmental organization worker with a South African accent and an ethos straight out of (Graham) Greeneland. “The question while you’re here is: Why would this person be honest? Why would any person risk that?”

Boris McGiver is very good as this soused truth-teller, and he, Charles Parnell, James Rebhorn, and Owiso Odera also offer chilling portraits of ineffectual bureaucrats from various governments. These four actors are also saddled with a potentially unwieldy amount of exposition; Mr. Rogers assumes, probably accurately, that Western audiences will need a primer on the Hutu-Tutsi tensions, the history of colonial depredations in the region, and the involvement (or lack thereof) of United Nations forces.

That these mini-lessons don’t clutter the narrative too much is a testament to the efforts of Mr. Stafford-Clark, an acclaimed London director whose Stateside visits have become too infrequent. He moves his actors on and off the stage beautifully, and he has a veteran’s gift for making crisp sense of elaborate scenarios.

Take the early cocktail reception, in which the Exleys rub elbows with many of the diplomats and leaders who will play a pivotal role in Jack’s search. Several languages are spoken at the event, the discourse includes references to Nietzsche and Montaigne — it’s all a bit overwhelming in its erudition, and yet a menacing thrum can be heard beneath even the most innocuous cocktail-party chatter. This blend isn’t easy to convey, and Mr. Stafford-Clark and his versatile cast pull it off effortlessly.

That undercurrent will ultimately engulf the stage, of course, and while Mr. Stafford-Clark and set designer Tim Shortall convey the enormity of the carnage with a pair of final images (one chilling, one slightly obvious), Mr. Rogers encapsulates the terror with a fateful convergence of the Exley and Gasano families. Characters we have come to know and recognize make split-second choices with horrific consequences. The results are abrupt, harrowing, and entirely believable.

Not for the first time, Mr. Rogers demonstrates the blinkered loyalties that have resulted in unimaginable suffering throughout Africa — and, not incidentally, have relegated these sufferings so far to the periphery of American attention. This is a vitally important message, and not even the occasional lump of undigested narrative can drown out the convulsive force of “The Overwhelming.”

Until December 23 (111 W. 46th St., between Sixth and Seventh avenues, 212-719-1300).