Sometimes even a 28-room mansion in the Hamptons isn’t big enough for two people.

At least not for Edith and “Little” Edie Bouvier Beale, the two-of-a-kind women immortalized in the 1973 David and Albert Maysles documentary “Grey Gardens.” The Beales spent their days in a shared bedroom with a hot plate, as Edith accompanied herself on old records in a cracked soprano while the 56-year-old Edie blamed Edith for her blighted life. They also happened to be Jackie Kennedy’s aunt and cousin. A tousle-haired teenager named Jerry occasionally stopped by their cat- and raccoon-infested estate and ate corn on the cob.

Does this sound like the makings of a musical or what?

Yes, as it turns out, and quite a good one. Songwriters Scott Frankel and Michael Korie, along with bookwriter Doug Wright, have taken “Grey Gardens,” a chronicle of shambling fabulosity that is practically a sacred text within gay culture, and turned it into a haunting, tuneful rumination on fading glamour and fraying nerves. They have added new facts and made wild suppositions about the Beales’ lives, which is perfectly in keeping with their subjects’ self-mythologizing natures.

The biggest gamble they and director Michael Greif have taken is to expand the story backward, constructing a semi-fictionalized back story to explain who the Beales are and what they’re doing in Grey Gardens, their Hamptons mansion. What sounded like a skittish move, an unnecessary respite from the recriminations and vitriol and flatout nuttiness of that Beckettian house, ends up adding insightful context for “Grey Gardens” veterans while making it more accessible to neophytes. Only a failure of confidence on the writers’ part, a need to bridge the two acts with a glut of extra bodies in the later scenes, prevents this gambit from succeeding entirely.

It opens with a quick glimpse of the 1970s Beales hammering away at each other – Edith (Mary Louise Wilson) croaking along to an old recording while Edie (the sublime Christine Ebersole) natters on about the past: “Let’s face it; time is a thief. You can’t get it back.”

And suddenly, they get it back. Or, rather, the audience does, as Allen Moyer’s deft set opens up to a fateful summer day in 1941 that sealed both women’s fates. (Mr. Greif cannily shifts Ms. Ebersole to the role of Edith for this act, while Sara Gettelfinger takes over as the tempestuous young Edie.) It is the authors’ conceit that both women lose their men on the same day: Joe Kennedy Jr. breaks off his engagement to Edie after a telegram arrives from her father, who has headed off to Mexico with his new girlfriend, abandoning the family.

Despite these grim circumstances, the act plays out like a sparkling period comedy with just enough intimation of the misery that lies ahead. Several of Mr. Korie’s lyrics have a splash of Cole Porter’s pizzazz, including the list song “Peas in a Pond” and a paean to Edie called “Body Beautiful Beale” (“Goddess beauteous / Gams to gluteus”).

Even better is “Drift Away,” a plaintive ballad that brings to mind some of Billy Strayhorn’s splendid torment, and a duet for Edie and Joe (Matt Cavenaugh) that deftly charts their increasing disillusionment. (Ms. Gettelfinger suffers a bit in comparison to Ms. Ebersole’s indelible Edie of Act II, but her buoyant performance shows the appropriate cracks beneath the surface.) Messrs. Frankel and Korie also have a knack for the operetta-era trifles that Edith plans to sing – nine of them, thank you – at the engagement party.

Messrs. Frankel, Korie, and Wright are clearly aware that they’re walking on hallowed ground as far as many gay men are concerned. And their solicitude occasionally takes on a shade of condescension, as when a visiting prepubescent Cousin Jackie (Sarah Hyland) goes moony-eyed at the idea of the White House decorated in chintz. This is what is known in politics as “mobilizing one’s base.”

But Mr. Greif’s direction and Mr. Wright’s book slip numerous glimpses of the future into the froth, from the two women shouting over each other to Edie’s fondness for Robert Frost to Edith’s blinkered optimism. “Soon you’ll be miles apart. And better for it!” announces the domineering Beale patriarch (John McMartin, as charming and effective as ever), echoing what would become Edie’s mantra. And after the engagement is clearly doomed, Edith comforts her daughter about the idea of solitary life: “There’s a lot to be said for living alone. You get to be a real individual.”

Sure enough, they don’t get much more “individual” than the 1973-era Edie on display as Act II begins. Ms. Ebersole, who transcends mimicry to deliver a devastatingly funny and heartbreaking performance as a grown-up Edie, gives a musical summation of the cracked do-it-yourself fashion sense that has inspired so many designers: “Re-invent the objet trouve / Make a poncho from a duvet / Then you can be / With Cousin Lee / On Mister Blackwell’s list.” (Edie would undoubtedly approve of costume designer William Ivey Long’s dependably riotous embellishments.)

With the exception of guerrilla pranksters like Michael Moore, today’s documentarians stay discreetly out of sight. The Maysles brothers found themselves in a different situation in 1973: The Beales, particularly Edie, saw them and their camera as an ideal audience for their mad performances, and they’d acknowledge the brothers by name. In time, the Maysles were accused of exploiting what was almost certainly mental illness in one or both of the Beales.

Mr. Wright lets Edie acknowledge the presence of outsiders in Grey Gardens, but this time the entire Playwrights Horizons audience are the interlopers. The guilt of intrusion has been diffused and collectivized, simultaneously implicating us all and (relatively speaking) letting us off the hook.

Unfortunately, the contemporary material isn’t quite as strong as the flashback. Mr. Wright wisely uses large chunks of the actual Beales’ dialogue, and the Proustian use of Edie’s wall of tchotchkes is a masterstroke, but he struggles to find a satisfying ending. Messrs. Frankel and Korie have a harder time finding a consistent musical vocabulary for the period, shifting from 1970s pop to string-heavy dissonances to refracted period songs, although they and Mr. Greif bravely permit their stars to sing them with the Beales’ less-than-sterling vocal stylings.

Ms. Ebersole and Ms. Wilson reward their trust, giving the finest pair of performances on a New York stage today. The Beales clearly loved the sound of their own voices, and these two actresses simultaneously play to themselves, each other, and the audience with consummate craft and wrenching humor. The abrupt shifts from gallows humor to keening sadness, from affection to loathing, are handled matter-of-factly: For Edith and Edie, these rancorous, contentious days are no different from any other.

Other than occasional visits from Jerry (Mr. Cavenaugh again), the Beales see virtually no one. So what to do with all those other characters, the cute little girls and the white-haired paterfamilias and so forth? Well, it turns out Edith and Edie aren’t the only characters slugging it out for decades and decades. Grey Gardens is haunted, or Edie has a backup chorus in her mind, or something like that. This is, to my mind, the only place where this “Grey Gardens” really goes wrong.

On several occasions – including Edie’s grotesque “military dance,” which the Maysles captured so memorably with one fixed camera shot – the whole rest of the company pops up for a little unnecessary oomph. Edie and Edith are who they are because they are all they have. Breaking up the oppressive solitude of Grey Gardens with a batch of triple threats is like plunking a kick line into “Waiting for Godot.”

The one exception to this comes in its sly rethinking of the film’s unforgettable Norman Vincent Peale sequence, when the two listen to Peale’s positive thinking radio program while an absentminded Edie examines her shoe and Edith blinks back tears. Perhaps because it’s the only real intrusion of the outside world in the film, this sequence opens comfortably enough into “Choose To Be Happy,” a gospel number led by Peale (Mr. McMartin) that triggers a necessary plot point.

But the Beales, both in real life and as depicted so skillfully here by Messrs. Frankel, Korie, and Wright, don’t need the help. There’s a lot to be said for not living alone, too. But there are limits.

Until April 9 (416 W. 42nd Street, between Ninth and Tenth Avenues, 212-279-4200).