“It’s a new old world to me,” sing mother and daughter. “It’s a new old world and / We are here.” The ladies refer to Florence, where they have traveled for a holiday. They might also be talking about “The Light in the Piazza,” which opened last night at Lincoln Center.

The musical is new because it’s a first-time collaboration between librettist Craig Lucas and composer lyricist Adam Guettel, who showed young promise with “Floyd Collins” nine years ago. It’s old because they have based their show on Elizabeth Spencer’s popular 1959 novella about Clara, an American girl who falls in love with an Italian boy, and her mother, Margaret, who has reasons for keeping the two apart. It’s “new old” because that’s the kind of fight Mr. Guettel and Mr. Lucas are waging.

That the battle has been fought and won countless times over the last century makes no difference; the need to fight it persists. Like Rodgers and Hammerstein, Kern and Sondheim, Mr. Guettel and Mr. Lucas share the implicit belief that the flimsy musical theater that surrounds them is not all the musical theater there is – that sincere artists can extract from the form’s motley amalgamation of opera, operetta, revue, and vaudeville not just a diverting evening, but also an experience of emotional weight and musical beauty. Audiences won’t leave their show with anything like the sense of revelation that greeted, say, “Show Boat.” But at a time when musical theater is dominated by overblown spoofs, runaway irony, and soul-crushing quasi-pop, the original and winning “Piazza” is welcome not least because it proves that Broadway isn’t entirely the province of licensing agents and Frank Wildhorn. Not yet, anyway.

For sheer loveliness, the pleasure imparted to eyes and ears, no show in town can touch what Mr. Guettel, Mr. Lucas, and director Bartlett Sher have managed here. Scenic designer Michael Yeargan reproduces Florence at what appears to be full scale. The Beaumont stage is about the size of a city block, and his sets use every cubic foot of it. The piazza where Clara meets handsome, non-Anglophone Fabrizio is dominated by a statue the size of Colossus. Towering arches glide thither and yon like icebergs. (Jonathan Butterell is credited with musical staging, but don’t expect high-kicking chorines: The real choreography here is executed by pieces of Mr. Yeargan’s set during scene changes.) At center stage occasionally appears the elegant haberdashery run by Fabrizio’s father, Signor Naccarelli.

The vast expanse puts some strain on what is basically an intimate story. Mr. Sher scatters ensemble players in distant quadrants of the stage: a guy with a cane, a gaggle of nuns, to no particular purpose. He also has the cast literally running from point A to point B, an unfortunate blocking tendency but one that at least saves us from watching entrances and exits all night. (Thanks to Catherine Zuber’s inspired costume schemes, never has a track team looked more fetching.)

Crucially, the score rises to fill the space provided. Mr. Guettel’s music slides from intricate piano-and-pizzicato figures to huge, lush strings. On one side, the romantic elegance of the harp; on the other, a flash of Hollywood, thanks to a gong. At times, Mr. Guettel aspires to opera, and the most successful numbers tend to be ones sung by world-class voices. Some of Fabrizio’s singing lies just out of Matthew Morrison’s considerable reach, but Sarah Uriarte Berry (playing his sister-in-law) has never sounded better than in the soaring “The Joy You Feel.”

Mr.Guettel’s score acknowledges that a man named Stephen Sondheim exists without attempting to channel him. He lacks the lyrical prowess of the master but echoes something in his tone, a kind of romantic chagrin. The moment that glows the brightest is “Let’s Walk,” a duet for Margaret and her not-quite-lover, Signor Naccarelli. It manages to be achingly lovely while carrying its own gentle propulsion: This is exquisite writing.

It’s a testament to Mr. Guettel’s music and the talent assembled by Mr. Sher that you so rarely notice the weakness of the show’s drama. Margaret tries to keep Fabrizio away from her daughter because Clara has a disability. (Among the show’s themes, Importance of Equestrian Safety falls just below Libidinal Potency of Italians.) As details of Clara’s condition come to light, the second act gets stuck switching between “the wedding’s on” and “the wedding’s off,” without building much emotional investment. Kelli O’Hara sings gorgeously as ever here, but doesn’t quite navigate Clara’s developmental slalom. To be fair, playing a 26-year-old who imparts sophisticated love songs despite having the faculties of a little girl may be beyond everyone. Speaking of unwise demands, sizable chunks of Mr. Lucas’s libretto are delivered in Italian. To sell this approach to the audience, the show makes its one self-aware gesture. It’s clever, but material this rich deserves better than clever. “I wish I spoke better Italian,” said my friend at intermission. “I wish I didn’t have to,” I replied. If “Piazza” doesn’t ultimately have the emotional payoff promised by its best moments, those moments still compel. They tend to feature Mark Harelik, sensational as Signor Naccarelli, and Victoria Clark, dazzling as Margaret. He gives the paterfamilias a debonair gravity, funny and involving without strain; she makes Margaret’s predicament thrillingly real, the deep center of the show. In narration, dialogue, and song alike, she conveys how Margaret is torn between loyalty to her husband, love for her daughter, and her own unhappy desires. Sometimes Ms. Clark is funny, mostly she’s moving, and every note sounds as if she was born to sing Adam Guettel’s songs. Here’s to more from both of them.

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