Unsatisfying as a show but musically extraordinary, "Floyd Collins," at the Signature Theatre, is based on the true story of a man trapped in a Kentucky cave for 17 days in February 1925, while above ground a media circus raged and roiled. The story had everything--sentiment (the poor trapped man!), horror (the dark, deadly cave!), local color (Kentucky hillbillies!). And to cap things off, Collins obligingly supplied the perfect unhappy ending by dying.

Collins's predicament and death have been dramatized once before, in the 1951 Kirk Douglas film "Ace in the Hole" (also known as "The Big Carnival"), which castigated the callous, scoop-greedy journalists who flocked like jackals to Collins's living grave. Adam Guettel and Tina Landau's musical isn't as focused (though, unsurprisingly, the most successful number, in old-fashioned musical comedy terms, is the reporters' blithe, heartless "Isn't That Remarkable?"). Landau's book has a sporadic quality, picking up and dropping characters and bits of story without developing anything.

This isn't necessarily the wrong approach for a dramatic parallel to Guettel's dark, plaintive, mysterious score. But it is the wrong approach for the script's realistic style, which features intense conversations and moral outrage right out of Arthur Miller. While the music whirls and shimmers like a ghost, the book clomps along earnestly.

Guettel's lyrics, straining for poetry, aren't up to his score, either. Dissonant and odd, then suddenly achingly romantic, the rich music seeps around the constricting words. (Musical directors Kimberly Grigsby and John Kalbfleisch bring out all the eerie clarity in Bruce Coughlin's orchestrations.)

Any tension the show might have depends on the audience's concern for Collins, which in turn depends on the audience's fully understanding what is happening to him and when. "Floyd Collins" gets very vague on this issue, and James Kronzer's set doesn't clarify matters.

You can't really tell exactly how Collins is trapped, or how hard it is to get to him, or whether the people who thought they could rescue him were correct or just fooling themselves. Since we don't know whether rescue is even possible, the arguments about the best method of freeing the trapped man have no dramatic force; they're just noise.

Director Gordon Greenberg has given the show a straightforward staging, trotting out the cliched characters--the evil businessman, the mad and mystical sister, the wise mother--and putting them through their paces. Neither he nor the actors seem comfortable when the scenes involve standing around and arguing or imparting information, but things come to life when the action is abstract--as in the phantasmagoric "The Carnival," Collins's vision of the media madness up above.

Rich Affannato is affecting enough as Collins, but he's not a particularly compelling singer. Except for Garrett Long, who plays Floyd's crazy sister and has a clear, exquisite soprano, no one in the cast is--and a few voices are waveringly inadequate. And none of the acting overcomes the thin obviousness of the writing.

Kronzer has designed an ordinary-looking cave that goes with the flat words and flat characters, though the music seems to demand something awesome, some cavern out of "Peer Gynt." The presence of the orchestra up behind the cave entrance is often disconcerting. Jonathan Blandin's spooky, dreamlike lighting provides some of the dread the material seems to demand, as does Brian Keating's sound, which echoingly evokes the abyss.

Collins comes to a kind of peace before his death, of course, as characters in American films and drama inevitably do. No one but a villain ever goes out screaming in despair. It's disappointing that such a musically daring work has such a timid and conventional ending.

"Floyd Collins" can simultaneously get on your nerves and entrance you, as if it were a dream with an irritating surface narrative laid over a sublime latent meaning. You can find the whole idea of the mystical idiot girl teeth-grittingly annoying, for example, yet still be stunned when she wanders against a green sky singing her strange songs.

The night I saw the show, some people left at intermission, and I understood why. But I also understood why this odd musical has won the awards it has and made so many critics swoon.

Floyd Collins, music and lyrics by Adam Guettel, book and additional lyrics by Tina Landau. Directed by Gordon Greenberg. Choreography, Ken Roberson; costumes, Anne Kennedy; props, Christine Kelly. With Will Gartshore, Patricia Pearce Gentry, Jason Gilbert, David T. Grimes, Ty Hreben, John J. Kaczynski, Dwayne Nitz, Michael Sharp, Tom Simpson, Scott Sedar, Jim Zidar. At the Signature Theatre through Feb. 10. Call 703-218-6500.

CAPTION: The shortcomings of "Floyd Collins" are redeemed by its music.