“Grey Gardens” may have moved from the cramped quarters of off-Broadway to roomier digs, but no house is spacious enough to keep Edith Bouvier Beale and her daughter, “Little” Edie, out of each other’s thinning hair. If a 28-room, cat- and raccoon-infested Hamptons mansion wasn’t big enough for this unforgettable tandem, what chance does Broadway have?

Working from the superb 1975 documentary by the Maysles brothers documentary that immortalized these two iconic, possibly mentally ill recluses (who happened to also be Jacqueline Kennedy’s aunt and cousin) in 1975, composer Scott Frankel, lyricist Michael Korie and book writer Doug Wright have furnished Edie and Edith with a haunting, tuneful rumination on fading glamour, fraying nerves, and the transitory but vital rewards of self-deception.

They are hardly the typical ingredients for a Broadway musical, and “Grey Gardens” is almost never typical. Even when its ambitions outstrip its execution, it succeeds in its central, seemingly paradoxical goal: to create a grownup show about two women for whom growing up is unimaginable.

The biggest gamble the authors and director Michael Greif take is to expand the story backward and explain how this has come to pass, adding what comic-book fans call an “origins story.”This time our larger-than-life, conspicuously attired heroine is contaminated not by a radioactive spider or gamma rays but by her mother.

After a brief glimpse of the middle-aged “Little Edie” Beale (Christine Ebersole, in the performance of a lifetime) and her mother, Edith (the superb Mary Louise Wilson), hammering away at each other circa 1973 — Mr. Wright has augmented the prologue slightly, perhaps with an eye toward a wider audience that might not know the film — the action shifts to a fateful summer day in 1941 at Grey Gardens.

The authors, extrapolating from real-life events with a devil-may-care brio that the self-mythologizing Beales might have admired, have created a scenario in which this combative yet oddly nurturing relationship is cemented.

Preparations are in full swing for a party celebrating 23-year-old Edie’s engagement to the politically ambitious Joe Kennedy Jr. (Matt Cavenaugh) before he heads off for a resume-polishing stint in the Navy. The glamorous Edith threatens to dominate the party with a protracted vocal recital, an act that her scornful father, J.V. “Major” Bouvier (the reliably effective John McMartin), dismisses as the desperate caterwaulings of “that most pitiable of creatures — an actress without a stage.” (Ms. Ebersole switches to the role of the elder Beale in this act, while a Broadway newcomer named Erin Davie takes over as the tempestuous young Edie.)

Before the day is over, Edith’s machinations and Edie’s somewhat sordid past result in the dissolution of the engagement. Edith’s marriage is crumbling at the same time, and Messrs. Wright and Greif cunningly show how this may have affected Edith’s destructive actions, setting in motion the resentments and recriminations that fuel the second act.

History buffs will recall that Joe Kennedy would not return from the war; “Grey Gardens” buffs will relish the irony of lines like “Soon you’ll be miles apart! And better for it!” in light of what lies ahead for Edie and Edith. But despite these sobering facts, Act I plays out like an effervescent 1940s musical comedy with just enough intimations of the suffering that lies ahead. Several of Mr. Korie’s lyrics, particularly in the charming list song “Peas in a Pond,” have a splash of Tin Pan Alley pizzazz, while Mr. Frankel’s finest moment comes with the plaintive, Billy Strayhorn-esque ballad “Drift Away.”

The net result of the extensive Act I revisions in terms of quality is negligible. A toothless new duet for Edie and Joe called “Goin’ Places” substitutes razzmatazz for the deft one-upsmanship that punctuated the original song in its place, and the Major has an ineffectual new song that addresses the Bouvier women’s emphasis on marrying well. But the streamlined book adds a welcome bit of ambiguity to Edith’s motives, and “The Girl Who Has Everything,” an introductory ballad that both honors and condemns Little Edie (“The crowds and the clamor / Aroused by her glamour / Will fade like the echo of a chime”), marks a pronounced improvement over its predecessor.

The recasting of the younger Edie also pays off. Sara Gettelfinger, who created the role off-Broadway, conveyed a robust, almost brittle self-assurance that curdled only after great provocation. Ms. Davie, by contrast, brings a weaker, more tremulous strain to the role; this Edie suspects ruination is perpetually lurking around the corner. She seems to know almost as much about her cloistered future as we do, and the realization is poignant to watch. Ms. Gettelfinger’s portrayal may have provided bigger fireworks, but the current portrayal works better as a beacon of what lies ahead.

The shambling fabulosity of Little Edie, however, remains unbowed once we meet her in Act II, in the form of Ms. Ebersole. Edie’s do-it-yourself fashion sense is as maniacally outre as ever: “Re-invent the objet trouvé./ Make a poncho from a duvet. / Then you can be / With Cousin Lee / On Mister Blackwell’s list,” she sings in the memorable “The Revolutionary Costume for Today.” (Costume designer William Ivey Long’s riotous embellishments could start a revolution of their own.)

“Grey Gardens,” while fizzy and fun in Act I, gets its emotional heft from the subsequent descent into decrepitude. This stems in part from Mr. Wright’s shrewd adaptation of the film — his new material rests very comfortably alongside the swaths of dialogue lifted verbatim — but far more from the glorious performances of Ms. Ebersole and Ms. Wilson, who make up the finest onstage pair musical theatre has seen in years.

Never mind their uncanny portrayals of the Beales or the fact that Ms. Ebersole’s transformation into the blinking, paranoid Edie immediately follows her flawlessly chic take on the 1941 Edith. While Ms. Ebersole has certainly deserved the extensive praise she’s received, Ms. Wilson matches her line for line and note for note in Act II. Their confrontations pulse with a punchdrunk weariness that one rarely finds in a Broadway play, let alone in a musical; the fact that the two women also sing marvelously seems almost like a bonus.

Unfortunately, the second act’s structural flaws, most notably an overreliance on the ensemble, have yet to be addressed. Ghostly Bouviers and Kennedys still lurk around the mansion to lend harmonic support throughout Act II, which remains a grave mistake. Tossing a chorus into, say, Edie’s dazzlingly arrhythmic “military dance” — a scene that the film captured with one fixed camera shot — grievously undermines the oppressive sense of solitude that pervades every one of those 28 rooms. And Messrs. Frankel and Korie’s 1973 material, while often pleasing to the ear (the ballad “Another Winter in a Summer Town” is particularly strong), lacks the stylistic cohesion of their ’40s period pastiche.

Edith is more accurate than she knows when she consoles a despondent Edie in Act I: “There’s a lot to be said for living alone. You get to be a real individual.” After 25 rancorous years of living alone together, the Beale women are about as “individual” as they come. Are they tragic heroes or just mock-heroic tragedies? To its credit, “Grey Gardens” doesn’t answer these questions. Its creators and their two wonderful stars are content to present the straitened, compromised, unexpectedly funny lives of two tattered legends who had pretty close to everything a long, long time ago.

Open run (219 W. 48th St., between Broadway and Eighth Avenue, 212-239-6200).