BOSTON, MASS.- “It may surprise you,” Jackie Siegel says, “but we are not old money.”

Surprise us? Probably not, but there were some context clues. Such as that she utters these words while dressed to the pink and sparkly nines, holding a tiny, fluffy dog and perched in the lap of her decades-older husband, David, whose capacious, ornately gilded chair suggests delusions of royalty.

So does their home construction project: a 90,000-square-foot house modeled on the Palace of Versailles (because, you know how it is, their current 26,000 square feet are feeling cramped) and built, Jackie tells us, “in the most beautiful place in the entire world — Orlando, Florida.”

The audience at the Emerson Colonial Theater in Boston got a good guffaw out of that on Thursday’s opening night of “The Queen of Versailles,” the surprising and frequently excellent new musical starring an utterly disarming Kristin Chenoweth and co-written by her “Wicked” composer-lyricist, Stephen Schwartz.

Then again, it may be a sort of genius to stage the world premiere of this show, which has already announced a Broadway run next season, in a city that is fundamentally identified with the origins of this nation and constitutionally disposed to adore old money but turn its nose up at vulgar flash.

Because “The Queen of Versailles,” based largely on Lauren Greenfield’s 2012 documentary of the same name, is as much an exploration of the seamy underbelly of the American Dream as is the very different new musical “Gatsby,” wrapping up its own world premiere across the river in Cambridge. (More on that momentarily.) Like F. Scott Fitzgerald’s Jay Gatsby, Jackie Siegel came from not much at all, left her humble roots behind and — with a husband (F. Murray Abraham, in terrific form) whose beginnings were similar — reinvented herself on a scale so over the top that strangers can’t help gawking.

Mind you, fans of reality television — which the real Jackie Siegel has embraced since the documentary — will be more versed in her life and persona than those who shy from that genre. But the “Versailles” musical, which has a book by Lindsey Ferrentino and direction by Michael Arden, stands on its own, no previous familiarity required.

It is a story of excess and fame, of a fortune vanished in the 2008 economic crash and then regained. It is also (this may be a spoiler) a story about the death of a teenager: Victoria (Nina White, poignant and appealing), Jackie’s firstborn, a compelling adolescent presence in the documentary and the only one of her seven children seen in the musical. The show is still finding its way toward telling that essential strand of Jackie’s biography without throwing off the dramatic equilibrium.

Act I fizzes entertainingly along, striking just the right tone, Chenoweth’s playfulness and charm endearing Jackie to us. And the show, which starts with and occasionally returns to the original Versailles, looks great. (Set and video are by Dane Laffrey, costumes by Christian Cowan, lighting by Natasha Katz.)

But in Act II, the tone flails, even descending briefly into sentimentality with a song called “Little Houses.” The cast is rock solid, and Chenoweth gives a fiercely intelligent performance, emotionally nuanced and deep. The narrative ground beneath the actors is unsteady, though. The darkness has — for now, at least — thrown the show’s creators off.

Readers of “The Great Gatsby” know to expect darkness in any serious adaptation. That’s part of the reason the frothy musical of the same name currently on Broadway feels so hollow. But the new “Gatsby,” at the American Repertory Theater in Cambridge, is another creature altogether.

With a book by Martyna Majok and a score by Florence Welch (music and lyrics) and Thomas Bartlett (music), this show is smart, inquiring and fluent in the subtleties of class. Its arty chic lets you know it’s a Rachel Chavkin production, the ambience flickeringly reminiscent of her work on “Hadestown” and “Natasha, Pierre & the Great Comet of 1812.”

Majok, who won a Pulitzer Prize for “Cost of Living,” has said that she sees Gatsby as a working-class character. What’s astonishing is how clearly she and her collaborators draw parallels between Gatsby (Isaac Powell) — enmeshed in an affair with Daisy Buchanan (Charlotte MacInnes), the wealthy love of his life — and Myrtle Wilson (Solea Pfeiffer), the working-class woman risking her own marriage for an affair with Daisy’s spoiled creep of a husband, Tom (Cory Jeacoma).

In this telling, Gatsby and Myrtle want the same thing: for their illicit liaisons to turn legit, granting them a kind of stability and acceptance that they have never known. When Gatsby gazes across the bay at Daisy’s house, that’s the beacon that beckons him.

On Mimi Lien’s thrilling sculptural set — a silvery wreckage of cars in a mountain of industry and ruin — the cast is almost uniformly strong. But Pfeiffer received the biggest applause the night I saw the show, and in what version of this tale has Myrtle ever gotten to be more than a shrill and tacky tart? She does here.

Admirers of Majok’s work might have expected her to pay close attention to Myrtle, but this layered, sympathetic take on her is powerfully transformative. One of the show’s most stunning moments is when Tom doesn’t hit Myrtle as he does in the novel, doesn’t break her nose, but instead does something at least as violent to her soul. He says to her, in front of a room full of people, “Marry you? You’re dreaming,” and uses an expletive to amp up the viciousness.

Daisy, too, here has more than the customary agency, even if she sometimes makes her decisions — like her choice to marry Tom, in the arrestingly fraught wedding-day song “I’ve Changed My Mind” — from a position of vulnerability and self-destruction. And George (Matthew Amira), Myrtle’s sweet and disappointing mechanic husband, often almost a background character, is thoughtfully and effectively fleshed out.

The score is frequently lovely, propulsive both musically and narratively, and there are some real standout numbers. Gatsby’s gambler mentor, Meyer Wolfsheim (Adam Grupper), gets one, delightfully.

Gatsby’s part of the story, though, is out of whack, partly because of the lack of sexual chemistry between Gatsby and Daisy. Powell has a gorgeous voice, but he errs on the side of recessiveness, not exuding the magnetism Gatsby needs. Why Daisy and her mensch of a cousin, Nick (Ben Levi Ross), are so drawn to him remains a mystery.

The show has yet to figure some things out, like how to balance innovation with faithfulness to the novel’s bones, and how to populate the set to make the party scenes at Gatsby’s house look sufficiently crowded. (The ensemble, cleverly clad by Sandy Powell, has a definite netherworld vibe.)

With the grace of Sonya Tayeh’s choreography, this production absolutely nails the final line of the novel and the final moment of the show. And Fitzgerald’s phrase “vast carelessness” is, perfectly, a lyric now.

He ‘used it to describe the behavior of Daisy and Tom. But it’s indelible because the habits of the obscenely rich tend that way in any age.

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‘Gatsby’

Closed Aug. 3 at American Repertory Theater, Cambridge, Massachusetts; americanrepertorytheater.org. Running time: 2 hours, 40 minutes.

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‘The Queen of Versailles

Through Aug. 25 at the Emerson Colonial Theater, Boston; queenofversaillesmusical.com. Running time: 2 hours, 45 minutes.

This article originally appeared in The New York Times.