When we get our first glimpse of Tyne Daly in Terrence McNally’s new drama “Mothers & Sons,” she’s wrapped in a luxurious fur coat. It’s a nice garment, even if it was bought at a second-hand shop, but it isn’t going to protect her from the chill inside an apartment high above Central Park.

As the curtain rises on the playwright’s 20th Broadway show, Daly, as widowed Texan Katharine Gerard, is standing shoulder-to-shoulder with Cal Porter (Frederick Weller). This is the first time Katharine and Cal have seen each other since the memorial service for Andre—her son, and Cal’s one-time partner—who died from AIDS complications 20 years earlier.

In the ensuing decades, Cal’s life has hurtled forward. He met Will, a man 15 years younger (Bobby Steggert, of “Big Fish”); they married, an option that didn’t exist when Cal and Andre were a couple; and they’re raising a child, a notion Katharine can’t wrap her head around. She remains mired in the past, unable to accept basic facts of her son’s life and weighed down by anger, at whoever infected Andre with HIV, and even at Cal, who she believes may somehow have turned her son gay.

Katharine has made an unscheduled stop at Cal and Will’s apartment to return Andre’s journal. Neither she nor Cal has ever had the composure to open it, and when they finally do, toward the end of McNally’s 90-minute play at the Golden Theatre, it makes for some haunting moments.

With “Mothers & Sons,” McNally (“Master Class,” “Love! Valour! Compassion!”) has again crafted a narrative that could not be more particular to time (the present) and location (the progressive Upper West Side). This time, it’s a story rooted in optimism, and one that manages to look simultaneously over its shoulder and straight ahead.

Daly gives an exquisite performance as a lonely, suicidal woman desperate to imagine a life her son might have led. She’s jolted when a nervous Cal returns from a hallway holding a poster of her son, from the time he performed “Hamlet,” because she is seeing a ghost. (“Mothers & Sons” is an expansion of McNally’s 1988 drama “Andre’s Mother.”)

Weller, the one-time regular on TV’s “In Plain Sight,” has fast become one of New York’s most useful stage actors, with underplayed turns in the past year at City Center (“I’m Getting My Act Together”) and Off-Broadway (“Reasons to Be Happy”). His Cal is a man who knows he dodged a bullet, and thus is all the more grateful for the gifts he’s been granted later in life.

Together, Daly and Weller have dynamic chemistry, lurching from moments of mutual respect to moments of accusation, and back. Katherine and Cal each have their own version of Andre, but they share a deep affection for him—here, Cal is the surrogate in a conversation that would, under better circumstances, have transpired directly between mother and son.

Katharine and Cal may be the heart of “Mothers & Sons,” but it falls to Will, an aspiring novelist, to underscore the reason we’re here. Arguing how future generations will one day look back on the final decades of the 20th century, Will says: “First it will be a chapter in a history book, then a paragraph, then a footnote. People will shake their heads and say, ‘What a terrible thing, how sad.’ It’s already started to happen. …”

Steggert is unapologetic and audacious throughout, in a fine portrait of a man who grew up in a world where the first question on a date wasn’t inevitably: “So, have you been tested?” 

His smarts aside, Will is largely unlikable. As portrayed by Steggert, he comes off as entitled, though come to think of it, perhaps that's the point. Will expects to be happy however he chooses, and can’t understand this interloper—his husband's ex-lover's mother, for lord's sake—with her backward ideas. Will’s inability to empathize with Katharine makes him unsympathetic as a person, but it also makes him the very embodiment of "positive change." As Weller’s Cal eloquently notes about the prospect of fatherhood: “I never expected to be a father. Will never expected not to be one.”

That child, incidentally, is Bud, played effusively by Grayson Taylor, whose innocence and frankness overwhelm Katharine, and help to defuse an otherwise tense afternoon. We’re left with a vague sense that Bud, who sees room in this family for a grandmother, may be the one who can help pull Katharine out of her emotional rut.

“Mothers & Sons” isn’t a melodramatic play about remembering the dead. Mostly, McNally captures a moment of hope and promise. It’s the first time a legally wed gay couple has been portrayed on Broadway. We’re here to remember the men who McNally has always chronicled in his plays, yes, but mostly to look forward from this singular turning point.

I thought “Mothers & Sons” was fantastic, for how effectively it locks down this unique period of time that is 2014, in New York City, amid the explosive progress of the gay rights movement in the last handful of years. I hope it finds a broad audience. If you’re under 30, “Mothers & Sons” is a history lesson; if you’re older, it may feel like the sun on your face.

“Mothers & Sons,” with an open-ended run at the John Golden Theatre, 252 W. 45th St. Tickets: $59-$137. Call 212-239-6200 or visit Telecharge.com.

Follow Robert Kahn on Twitter@RobertKahn