You’d be hard pressed to come up with a more constricting theatrical premise than one that traps a man for 16 days in a cave, still harder pressed to imagine a theatrical event more spirit-freeing than Floyd Collins, the exhilarating musical Adam Guettel and Tina Landau have fashioned from that premise.

Even for Signature Theatre, a troupe that specializes in breathing fervor into problematic musicals, this show must have seemed daunting. Based on a true story that has all the messy complexity of real life, it boasts a soaring but jawbreakingly difficult score and covers geographical terrain tricky enough to have given director Gordon Greenberg and his designers fits.

Still, damned if everyone isn’t up to the challenge. Remarkably, for a show that has previously been regarded even by its enthusiasts as a work-in-progress, Floyd Collins is not merely a hugely satisfying success for Signature, it’s a flat-out triumph—at once searing and expansive, inspirational and shattering.

The evening’s eponymous young hero is a dreamer who in 1925 got trapped hundreds of feet underground while exploring a warren of caves near his family’s Kentucky farm. Despite an elaborate rescue operation, the real Collins died before rescuers could free him, though not before journalistic sensationalism had turned their efforts into a national, newspaper-selling obsession.

Seeking to capture both the flavor of this media circus and the spirit of the frightened, increasingly inward-looking man at its center, Guettel’s score begins with a rustic-sounding “Ballad of Floyd Collins” that morphs into a character-defining musical soliloquy as the title character plunges into the cave that will become his tomb. Floyd (an impetuous Rich Affannato) initially clambers in near-darkness over platforms and across ceiling girders at Signature, hemmed in by a tight spotlight while singing of the call he says he hears from the rock itself. Then he lets loose with a sort of yodel, waits a second, and beams broadly when it comes bouncing back at him in ever-receding echoes. He’s searching for the sort of massive underground cavern that he can turn into a tourist attraction, and those reverberations mean it’s nearby.

Soon, with dozens of echoes bouncing in counterpoint (the technical requirements are handled superbly), Floyd is singing a duet with himself, then an ecstatic round, and by the time he finds the cavern he’s been looking for, it almost sounds as if he’s leading a choir. Until, that is, a sudden rockslide puts an end to his celebration. In seconds, his legs are immobilized. Then his arms.

At which point, Guettel and his librettist and co-lyricist Landau bring the world above ground back into focus. In quick vignettes, Floyd’s family marshals a rescue effort, a mining company recognizes the publicity value in helping to extricate him, and impoverished locals, including his father, start selling curios to the curious. Alas, Floyd’s younger brother, Homer (full-voiced Will Gartshore), and a small-boned Louisville reporter named Skeets (Jason Gilbert, at once wry and terrified) are the only folks who can get near him in the tight limestone cave. When their attempts to free him prove unavailing, his bad situation turns dire, and the score (which has briefly turned conventional to deal with more conventional dramaturgy) catches fire.

Reviews of Floyd Collins stagings in New York and London have almost invariably suggested that, although the music is gorgeous, audiences may find it difficult to absorb. I’ll never understand why critics say things like that. Frankly, I can’t imagine anyone who’s ever hummed a Joni Mitchell song having any trouble with a score this rich in long, arching melodic lines. Nor will the songs distress anyone familiar with the theater music of Stephen Sondheim—which presumably includes most of Signature’s likely patrons. Like Sondheim, Guettel has a gift for soaring dissonance and is adept at cloning period styles—witness his Act 2 ditty “Is That Remarkable?” which briefly turns three male reporters into the Andrews Sisters.

The 34-year-old composer has also acquired—perhaps from his grandfather Richard Rodgers—near-perfect pitch when it comes to matching emotion to melody. I’m not the first to note that the tricky 20-minute soliloquy he gives his doomed Kentuckian in Floyd Collins has much in common with the “Soliloquy” Rodgers wrote for a doomed New England carnival barker in Carousel. And Guettel is no less savvy about the syncopation in a jaunty little cheer-up song like “Lucky,” which is sung by Floyd’s sister and stepmom (Garrett Long and Patricia Pearce Gentry, both terrific) before they know how grim Floyd’s situation is. For those who have wondered whether the post-Sondheim generation of musical-theater creators will ever match its mentor, this score (and Landau’s libretto, which leaps cleverly past plot points that don’t lend themselves to musicalizing) will be hugely reassuring.

So will Greenberg’s giddily inventive staging. The rescue and the media-feeding-frenzy aspects of the story obviously offer a director plenty to work with (as Billy Wilder proved in his acerbic 1951 film based on the incident, The Big Carnival), but Floyd’s interior musings are another matter. As Affannato plays him, with boyish earnestness and fading faith, he’s hardly a deep thinker. Nor are his dreams—of a girl with blue eyes, of a cave with box-office appeal—the sort that would set toes tapping in a conventional musical.

All the more remarkable, then, that Greenberg not only finds so many ways to illuminate Floyd’s limited frame of reference, but does so without getting all mawkish about his physical imprisonment. When the music opens up—as it does when Homer tries to distract Floyd in “The Riddle Song,” which may just be the most horizon-expanding theater song to come along since Sondheim’s “Someone in a Tree” a quarter-century ago—the director widens the frame to give the performances more room. And then, when his focus re-tightens to the stage equivalent of a close-up, the intensity is heightened to the point that he can find enormous emotion in whispers and head turns.

He’s aided by James Kronzer’s setting, which confines the title character in a girdered iron chamber that looks a bit like a carbon crystal, and which takes full advantage of Signature’s intimate, low-ceilinged auditorium by incorporating the theater’s roof beams into its maze of caverns. Also helpful is Jonathan Blandin’s alternately claustrophobic and ethereal lighting. For long stretches, Blandin’s effects and Brian Keating’s precisely calibrated sound design pretty much create the cave, allowing the director and his sharp cast to concentrate on the resonance of what happens in and around it. That resonance has everything to do with echoes—of other rescues, other media frenzies, other hapless dreamers, other families in crisis, and other musicals—but it can also take your breath away simply because the echoes come back so gracefully, or, in one exquisitely harrowing moment in the second act, don’t come back at all.

You’ll want to discover those echoes for yourself…and soon, so you’ll have time to catch them again before they recede forever. There’s a possibility of an extension, but I wouldn’t take any chances if I were you. CP