UPDATE Monday A.M.: Adds video clip.

Earlier: It’s been eight years since the Roundabout Theatre Company launched both its postage-stamp size developmental space on West 46th Street and the career of an extraordinary playwright, with Stephen Karam’s Speech & Debate, whose film adaptation wrapped a few weeks ago and is now in post-production for a 2016 release. Great timing, then for The Humans, which inarguably establishes Karam as one of the most incisive and, sure, human playwrights at work today.

Having graduated from the Roundabout’s sub-basement black box to its main off-Broadway stage, the Laura Pels Theatre, Karam remains stubbornly below the surface, excavating with surgical deftness the troubles and disappointments that one family struggles to temper with arch humor until, at last, the effort shockingly fails in ways we simply aren’t prepared for, and that leave us stunned.

The setting is a basement duplex in Chinatown. A young couple has recently moved in: Brigid (Sarah Steele, the breakout star of Speech & Debate and best known as Alan Cumming’s smart-ass daughter on The Good Wife)  and Richard (Arian Moayed, currently seen in Rock The Kasbah) have just moved in.

On first glance the digs look almost spacious; a spiral staircase joins the two floors, and there are arched entryways; Christmas lights twinkle while the furniture has yet to arrive. Look closer and you’ll note the check-list of compromises New Yorkers optimistically embrace, at least until reality sets in: Exposed pipes hang from the ceiling; gray meter boxes jut out from the walls. To get mobile phone reception, you have to cantilever yourself into the window frame. The windows themselves are barred and there’s no natural light to speak of, which is fine for the roaches that skitter across the floor of David Zinn’s set, an apartment that’d be ghastly anywhere else but here. Well, here too.

Every few minutes a thunderous pounding can be heard, which Brigid attributes, not convincingly, to the clueless woman who lives above them. Other basement noises intrude as well: the boiler switching off and on; the machines in the laundry room.

Brigid cheerfully indicates the good points to her skeptical parents Erik (Reed Birney) and Deirdre (Jane Houdyshell), who’ve driven in from Scranton with Erik’s dementia-addled mother Momo (Lauren Klein) to see the place. They’re also joined by Brigid’s sister Aimee (Cassie Beck), who lives in Philadelphia. It’s Thanksgiving.

Brigid and Rich feel lucky; she’s a struggling composer working two bar jobs to pay off her college debt; he’s working toward his master’s degree in social work, though he threatens to be a permanent student; a trust fund that won’t kick in till he turns 40 is looming in the not-too-distant future.

That news pricks up Erik’s ears. He has labored at at a private school for nearly three decades as a facilities manager, barely getting by; Brigid never lets more than a few minutes pass without reminding him that he’s never helped her out financially. Aimee is an emotional wreck, having just been dumped from the partner track at her law firm and her longtime girlfriend while contending with an ulcerated colon that will require surgery.

Sounds like fun, no? And yet for much of the play, these and other details are revealed with, dare I say it, sizzling humor and affection as the holiday meal gets underway. But the kids begin to notice that Erik has been hitting the beer supply with unusual determination. As the evening moves like nature from light into darkness, Karam and his incomparable director Joe Mantello (what a c.v.: Wicked, Other Desert Cities, The Last Ship) take this first class ensemble and us interlopers along a journey that’s part family drama and thriller: Those brain-rattling crashes grow louder as the cutting jibes turn to panic from an unexpected place. The play’s growing darkness links the Twin Towers attack of 9/11 through the devastation of Hurricane Sandy — a prism refracting the white light of their terror into a spectrum from the evil humans inflict upon ourselves to the force of nature we cannot control.

None in the cast could be bettered, but again I have to point out Steele. In addition to being a gifted comedian she has a remarkable ability to register alarm or anger or hurt with the tiniest change of expression, as unmistakable as it is fleeting.

The HumansThe Humans — the title tells us everything we need to know about the author’s empathy for these folks — is tremendously exciting theater. Karam’s fine sophomore play Sons Of The Prophet was a Pulitzer finalist; The Humans takes him to an even higher level, and it deserves a wider audience. You won’t see a better play this year.