It starts as just another family play on Thanksgiving. The aging dad worries about money, one of the two grown daughters moans about her student debt, the mother wheedles both of them on the benefits of marriage. Oh, and Granny -- called Momo -- has dementia and will probably not make another holiday.

But Stephen Karam is not just another playwright and "The Humans," his third play nurtured under the Roundabout Theatre's Off-Broadway wing, dares to assemble the usual bickering and raw nerves for what at first seems a hackneyed situation. In just 90 minutes of overlapping wit, tenderness and blistering brutality, however, Karam, director Joe Mantello and six expert actors -- including the masterly Reed Birney and Jayne Houdyshell as the mother and father -- walk us up to the edge of the abyss.

Karam, whose "Speech & Debate" inaugurated the Roundabout's tiny black-box series in 2007 and whose "Sons of the Prophet" was a 2012 Pulitzer finalist, has a Lebanese-American father and an Irish-American mother. "Sons of the Prophet," an emotionally wilder and more polished work, burrowed into the Lebanese side of the family.

"The Humans" is about an Irish-American family, also from working-class Pennsylvania, where Karam was raised. Both funny and tragic, like life, the play is also more scattershot in its plot points, with too much retelling of bad dreams and a few threads that feel dropped in without sufficient development.

More important, however, is the ultimate impact, the individual characters with their own honest agonies and the dread that creeps in with the understanding that, as the father says, "in the end, everything you have goes."

But Karam is marvelous at tragicomedy nuance, so all is hardly gloom and recriminations. At one point the resentful, blue-collar father tells Richard, daughter Brigid's privileged but depressed boyfriend Richard (Arian Moayed), that his family "doesn't have that kind of depression." Lesbian lawyer daughter Aimee (Cassie Beck) with the broken heart and the bad stomach retorts, "We just have stoic sadness."

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The family descends on the unfurnished new Chinatown apartment shared by Brigid (Karam veteran Sarah Steele) and her fragile trust-fund guy. The place (itself a genuine disorienting character by designer David Zinn) is a ground floor and basement "duplex" that, as the father notes, is too close to the downed World Trade Center and in a flood zone.

A mere mention of the terrorist attack still tends to be jarring in the theater, but there is a payoff. Karam imagines that, instead of people telling monster horror stories, they are the human horror stories. Thus the title. This one, if still a little wrought around the edges, is haunting.

WHERE Pels Theatre, 111 W. 46th St.

INFO $79; 212-719-1300; roundabouttheatre.org