NEW YORK — Deep, deep, deep into “The Inheritance” — Matthew Lopez’s elegiac reckoning for gay men in the era after the AIDS plague — a woman finally appears on the stage. She’s a mom played by extraordinary 89-year-old Lois Smith and, in a lengthy monologue fully six hours into this marathon, she recounts the wrenching details of her alienation from her homosexual son, who died of AIDS three decades earlier.

Smith’s delivery percolates with theatrical know-how. A lesser actor would think welling up would be called for. But Smith remains dry-eyed. She opens up the emotional space for an audience to savor her words and performance — and explore the boundaries of its own empathy and feelings of loss.

It takes all afternoon and evening to get to Smith’s tender soliloquy in the Barrymore Theatre, where the two-part play, astonishingly well-handled by director Stephen Daldry, marked its official opening Sunday. Moved to perhaps the greatest degree you’ll experience all day — and there are other reach-for-the-tissues milestones in this big Broadway epic — you do at this point ask yourself: Was there enough to this exhausting trip to make it worth it?

Well, ultimately, I’d say yes, but with some caveats. Because the playwright has created as his central character a contemporary young gay man of comfortable means for whom the epidemic is only a history chapter, the present-day suffering detailed in “The Inheritance” can feel manufactured and minor. (If deciding whether to marry a chilly billionaire played by John Benjamin Hickey is your most pressing problem, I know a lot of people who want your agony.) The memory of pain isn’t nostalgic, it’s a return to hell. So in the interludes when the drama most directly references the disease’s ravages — a riveting speech by an older gay man played by Paul Hilton; that touching requiem at the end of Part One — you’ll sense in the audible reactions of some of the people seated around you the firing-up of old, private hells.

That’s why “The Inheritance” feels powerful only by its associations with an earlier tragedy. And maybe why the best way to approach the well-acted production is to test-drive it, by investing only in Part One. Lopez’s literary inspirations — “Howards End” and “Maurice” by E.M. Forster, who’s also a character here — are reflected in the novelistic infrastructure of the piece. It’s not built as a cliffhanger, so returning immediately to Part Two doesn’t feel urgent. At the end of Part One, featuring a gathering I won’t describe but that recalls other sprawling eulogies for mass death, such as “Schindler’s List,” you’ll feel sufficiently filled with the themes Lopez adheres to all through both parts.

“If we can’t have a conversation with the past, what will be our future?” asks the play’s polestar, one pure-hearted Eric Glass, portrayed by moony-eyed Kyle Soller as sincerity incarnate. He’s an Upper West Side Jew in the Manhattan of 2015, a social activist living with Toby Darling (Andrew Burnap), a writer best described as hot trouble. Like Louis and Prior of Tony Kushner’s two-part “Angels in America” — which “The Inheritance” aspires to emulate — Eric and Toby reflect the mores, joys and sorrows of gay partnership of their era. With disease a receding worry and same-sex marriage a reality, Eric’s concerns are raising a family and passing on to the next generation an understanding of the struggle of the men who died needlessly and horribly before the virus could be contained.

Toby, in Burnap’s impressive, spasmodic bursts of nervous energy, loves Eric some but himself more. He is inexorably drawn to life’s heat, embodied by Adam (Samuel H. Levine), a younger man and budding Broadway star. Toby is the hedonistic yin to Eric’s conscientious yang, a pivotal tension in “The Inheritance” that doesn’t always feel like a big enough problem to hold you through 6½ hours.

The supplemental characters do help provide absorbing embroidery. Daldry’s mise-en-scene, executed by noted stage designer Bob Crowley, consists of a pale floor with a hydraulic platform and a set of black walls upstage that part to reveal simple, emblematic images: a house, a tree. Around the platform, the actors, in contemporary dress but mostly barefoot, sit or lounge, watching the play and divvying the narration among them. Hilton, who also portrays the late Forster (and is referred to as “Morgan”), speaks to the other characters; sometimes characters narrate their own actions, adding another layer of novelistic conceit.

Lopez’s supporting mainstays are the friends in Eric’s life, all trying to hold on to their gains as the Obama era flickers out and a more hostile America takes shape. Jordan Barbour excels as a doctor who as a black gay man no longer sees a future for himself in his own country, and Arturo Luís Soria wrings all the rewarding sunniness out of a man who both wants to settle down and keep the party going. The bigger featured roles are inhabited with soulful intensity by Hickey, Hilton and Levine, the last of these actors doubling as a traumatized prostitute with a brain of gold.

The complex question at the heart of “The Inheritance” seems aimed squarely at gay men Lopez’s age (he’s 42) and younger: How do they remain true to the sacrifices of their forebears, find fulfillment in spiritual and material achievement, and maintain solidarity through a common identity? It could take a whole book to work that out. And sometimes, this voluminous drama is too much like one.

The Inheritance, by Matthew Lopez. Directed by Stephen Daldry. Sets and costumes, Bob Crowley; lighting, Jon Clark; original music, Paul Englishby; sound, Paul Arditti and Christopher Reid. With Jonathan Burke, Kyle Harris. Part One: 3 hours and 20 minutes; Part Two: 3 hours and 10 minutes. Tickets: $39-$349 for each part. At Barrymore Theatre, 243 W. 47th St., New York. 212-239-6200. telecharge.com.