So fragile at the start that it seems likely to topple over before it has a chance to take root emotionally, Ricky Ian Gordon’s Sycamore Trees grows sturdier and more epic as it sings rapturously of six decades in the life of a boisterously neurotic New York family. With a composer at its center, a mom who was once a Borscht Belt entertainer, a gruffly uncommunicative dad, and three variously damaged or eccentric sisters, this intimate, personal musical is not merely precise in its evocation of a Jewish upbringing much like the author’s own. It straightens and unfurls until, by its final fade, it seems to have spread its protective canopy over the better part of 20th-century America.

The evening begins with a Pirandellian conceit in which the characters argue over who’ll set the scene, and how—call it Six Characters in Search of an Opening (with director Tina Landau hiding the evening’s nonfamilial Everyman for a few seconds so she can make that joke visually). “I have a story to tell,” says Andrew (Tony Yazbeck) knowing full well his family isn’t going to let him tell it without interference. “We all have a story to tell,” protests a sister, and within moments the song about 1940s family life that Andrew has coaxed from his mother (an effervescent Diane Sutherland) is being interrupted by her yet-to-be-born but theatrically cantankerous brood.

As Andrew tries to impose order, his intellectual elder sis Myrna (Jessica Molaskey) fusses with feather boas in the costume racks at the rear of a broad open stage, while underappreciated Ginnie (Farah Alvin) rummages for props and transcendental revolutionary Theresa (Judy Kuhn) hectors them all about getting in touch with their inner neuroses. Glowering from the sidelines is Mark Kudisch’s big, conservative, angry-at-the-world lug of a father, who wishes they’d all—the son who’s so disappointed him, especially—pipe down and let the love of his life sing her song.

That’s clearly not going to happen, so Andrew reboots—now with Mom and Dad doing a little soft-shoe about how they met—and this time the story has more momentum. Lilting, lovely songs about family travails alternate with bouncier ditties about family triumphs. An encounter with a gold-jacketed chatterbox of a real estate agent (Matthew Risch, the show’s versatile everybody-else) leads to a production number about moving from a too-small apartment to a house on Long Island, and as the family progresses through the decades (three in each act), the story finds its feet and the Pirandellian gimmickry fades.

There are a few plot bumps along the way, some of them pretty severe—Andrew’s disastrously fraught coming out at the age of nine, for instance—but for the longest while, nothing happens that Sutherland’s cheery matriarch doesn’t firmly believe she can smooth over by uttering a mom-ism (“Let’s eat!”) or reverting to her Catskills roots. At one point, to defuse tension at the dinner table (Dad’s furious; the kids are cowering), the actress turns a convulsively stammered joke about stuttering into a speech uproarious and anthemic enough that, music or no music, it deserves its own song title in the program.

Despite her spirited interventions, though, and the leading man’s best efforts to keep the storytelling bright, by intermission clouds are gathering ’round this clan. After the break there’s addiction, teen pregnancy, suicide, and HIV to deal with, and an already impassioned score heads, on more than a few occasions, into operatic territory. Gordon writes music that’s often described as hovering right at the intersection of Broadway and the art song: bubbly melodies without the conventionally catchy hooks that theater audiences—even ones raised on Sondheim musicals—tend to expect. That description is accurate enough as shorthand, but it doesn’t really do justice to his accessibly tuneful score for Sycamore Trees as it swoops and soars, marches and skitters from Dad’s ode to his pigeons, past passionate love ballads, through drug-addled patter songs, to a wrenching chorale about grace.

It helps that Gordon’s lyrics are at once conversational and evocative (“I dreamed of daughters and sons/Each of them a poem that jumps and runs”) and that the script he’s co-written with Nina Mankin has its nonmusical moments, too, including a hush-inducing climactic speech into which Yazbeck breathes such anguish that it effectively becomes an a cappella aria about death.

Landau’s staging—seemingly minimal but filled with delicate details (bruised veins suggesting addiction before we ever see a needle, a father’s reassuring touch prompting a quizzical look from his son’s lover)—is so smart that you’ll not be inclined to quibble too much about excesses here and there. Scenes in which Yazbeck’s Andrew and Molaskey’s Myrna get separately glassy-eyed on drugs, for instance, probably shouldn’t require him to vibrate on the floor or her to stagger upright but limp to the point of bonelessness through an entire song. Both performers are resourceful, but their bags of tricks aren’t bottomless.

Kudisch’s, on the other hand, just might be. His ferocious, working-class patriarch comes on snarling only to go all cuddly with those pigeons, croons to his wife in a sweet tenor purr before shifting to a gravelly baritone for his kids, turns cold as steel when disappointed by life but melts when his family’s disappointments exceed his own, and somehow makes every whiplash turn feel unforced.

The Signature production is Sycamore Trees’ world premiere, and it still feels like a work in progress—that soft start needs firming up; Act 2 needs tightening; balancing the three daughters takes too much time—but all of that’s fixable. The show’s heart, this family of misfits so worth worrying about, is already fully present. And Gordon’s unblinking embrace of their flaws, missteps, and insecurities is bracing in an age when musicals rarely take the real world seriously. It’s been a while since I’ve been so sad to hear a finale coming, knowing that a show was about to end.