Which is the driving force in a successful opera - the music or the story? Opera Philadelphia gave Jennifer Higdon's Cold Mountain its East Coast premiere Friday night at the Academy of Music, and the two-and-a-half-hour work, based on the Charles Frazier Civil War novel with a libretto by Gene Scheer, has some stirring stretches of music.

Higdon, the amiable 53-year-old Curtis Institute of Music professor, is a highly skilled composer. She has a fine ear for the vernacular, and brings an American sound - actually, several - to the score. Not once does she allow her orchestra to overwhelm the singers. She has a good feel for filling out something in a character's air, motive, or temperament by conjuring, for two seconds or the duration of a scene, corresponding melodic material or orchestral texture.

There is, however, in opera - if it is really about the music - the question of pure musical inspiration. I'm not speaking of whether or not the piece has tunes you can whistle. But does the music have something to say even completely divorced from the story and visuals? Does the music make you feel or think something that the words alone could not? There are individual arias and ensemble scenes within Higdon's piece that reach these heights. But there's also a lot of what sounds like filler, passages in which musical inspiration gives way to the utilitarian job of advancing the plot or using up all the words.

And so while in many ways, Cold Mountain the opera does a serviceable job of representing the book, or at least the bones of the plot, it doesn't always add up to a great piece of music.

The cast, led by conductor Corrado Rovaris, is uniformly fine and often much more than fine. Jarrett Ott, who replaced Nathan Gunn relatively late in the game, seemed vocally and dramatically the right man for the part all along. And for those who didn't show up for their love of contemporary music alone, Opera Philadelphia did what it does so well, cushioning the work in stunning visuals. Rarely do set and lighting design align as beautifully as those of Robert Brill and Brian Nason, respectively, surrounding characters in light, shadow, and the brawny planks of a central structure that perfectly signals the feel of a life chaotic and askew. War changes everyone into someone else, one character tells us, and the stagecraft is often the means for getting there.

You didn't miss the skies of Santa Fe, where the opera was premiered in August. At the end of the first act, projections fill the entire Academy auditorium with stars, and a full moon appears. Here is Higdon at her best, with all of the creative forces coming in behind her to do what only opera can. The story's two principal characters, W.P. Inman and his beloved Ada, are about to be separated by war. He is a Confederate soldier promising to return. The music is warm and wondrous. For a few minutes, life's luster seems to bring the audience and story together into the same space.

Similar moments of artistic clarity were numerous. Ada Monroe is a city girl left alone during the Civil War to tend Black Cove farm, and she takes on a partner, Ruby, a hard-knocks survivor who teaches her all about nature. There's a scene in which Ruby is telling Ada how to decipher the sounds of nature. "Until you can tell the sound of the poplar from the oak, you haven't started to know a place," Ruby sings. There are times when Higdon's penchant for writing literal responses to the text - someone sings of a clock, so here's the sound of a clock - can grow cloying and predictable. But what makes her musical reply so wonderful here is that the rush and rustle of the strings are so subtle and sophisticated they suggest something more about the spirit of the moment than mere text-painting.

The orchestra is sometimes challenged by the score, making this Cold Mountain another artistic notch in the ensemble's growth. Frazier's alternately sweet and brutal story is complex and episodic, and the cast, directed by Leonard Foglia, is large. It was startling to encounter a voice as major as Marietta Simpson's in a role as minor as that of Lucinda, a runaway slave; she was potent as ever. Ott had the great gift of humanity, his voice noble and strong, those same traits perfectly mirrored in his Ada, sung by Isabel Leonard. The role of Ruby is a nice, unlikely surprise, and in both her vocalism and body language, Cecilia Hall had youthful swagger. Jay Hunter Morris, as Teague, the Civil War deserter bounty hunter, was a delicious menace.

Whatever the individual achievements of this opera, the most significant one went unstated. Opera Philadelphia has figured out a way to keep creating new work in a tough climate - and, with this piece, in a venue as large as the 2,900-seat Academy. That gives contemporary opera in the city a role that is unexpected, and pretty grand.

Additional performances: Sunday, and Feb. 10, 12, and 14. At the Academy of Music, Broad and Spruce Sts. Tickets are $19-$249. Information: 215-732-8400, www.operaphila.org.