Fans of Andrew Barth Feldman, take note: The rising stage and screen star strips down to his skivvies, if only for a few moments, during the world-premiere production of Joshua Harmon’s new play, “We Had a World.”

Mr. Harmon and director Trip Cullman are not exploiting Mr. Feldman’s youthful pulchritude; they’re signaling that the following one-act piece will be as nakedly personal as anything the playwright — whose previous efforts include such witty and poignant works as “Bad Jews,” “Significant Other,” and the harrowing “Prayer for the French Republic” — has ever delivered.

Mr. Feldman plays Joshua, also a playwright, whom we follow from his childhood in the 1980s and ’90s to a few years ago, as his latest offering, “Skintight” — the name of a play by Mr. Harmon, produced off-Broadway in 2018 — is approaching rehearsals. Yet Mr. Harmon’s career, or rather the fictional Joshua’s, is not the focus of “World”; the new play is concerned with a subject that is typically even trickier and more consequential: family.  

We meet Joshua’s grandmother, Renee, played by an ideally cast Joanna Gleason, as she is dying of cancer. She has a final request for her grandson: that he write a play about their clan, and that he “make it as bitter and vitriolic as possible.” 

Of course, as anyone familiar with Mr. Harmon’s work could predict, “World” is not defined by vitriol. The play’s one other character, Ellen, Joshua’s mother and Renee’s daughter, certainly harbors some bitterness — relayed by Jeanine Serralles, in a performance that’s by turns hilarious and touching — though as we gradually learn, she’s got good reason for it.

Yet for all the barbs exchanged, especially between Renee and Ellen, the play’s spirit is generous and forgiving, and its tone, as the title suggests, is unabashedly nostalgic, even elegiac. From a young age, Joshua worries about climate change; as an adult, he points to the dearth of snow in New York and speaks wistfully of rainforests. “The world doesn’t seem, like, long for this world,” he tells Renee. 

But the world that’s slipping away here is not just one of forests and glaciers. Using a specific milieu that will be familiar to many New Yorkers, Mr. Harmon evokes universal shifts, away from a culture where women of a certain age enjoy leisurely phone calls rather than quick Zoom chats with their grandchildren — and, if they’re lucky enough to live nearby, take them to the movies and out for ice cream.

Renee, a Brooklyn native who lives on the Upper East Side, also brings a pre-adolescent Joshua, who like many suburban kids views the city as a shiny panacea, to an exhibit titled “Pubic Hair on Soap” and a production of “Medea” starring Diana Rigg. After the latter, the boy who will become a theater artist gushes, “Nana does not know it — neither do I — but I am 10 years old, and she has just changed my life.”

As “World” progresses, though, we see more and more of Renee’s darker side, the part of her that has caused Ellen suffering throughout her life and has helped drive a wedge between Joshua’s mother and her sister, Susan, whom we never meet but who doesn’t emerge in an especially flattering light, either.

Ellen is, in fact, the real heroine of “World,” a working mother who zealously protects her children and begrudgingly keeps giving her own mom more chances, even as they all test her patience. Ms. Gleason’s Renee, who can be funny and even delightful one moment and then send a little chill down your spine, lets us better understand both her forbearance and her frustration.

Mr. Feldman, likewise, makes Joshua deeply endearing as he conveys his ambivalence. “I wish I saw the world as black or white,” he says toward the end. “But the truth is more complex. Women who should not have been mothers can make very compelling grandmothers. Devoted mothers can raise ungrateful sons.”

In truth, “We Had a World” brims with Mr. Harmon’s gratitude; it may be the most refreshing feature of a play that’s as courageous in its fashion as it is disarmingly sweet.