The rather puzzling come-on for The Gershwins’ Porgy and Bess reads: “The masterpiece reborn as a musical.” Can’t musicals be masterpieces? (Of course they can.) If not, the slogan would seem to suggest some essentially diminished offering, as if what was once masterful about George and Ira Gershwin’s 1935 landmark — a work that in straddling genres pretty much defies them — existed no more.

A comparable haziness hangs over the results at the Savoy, in a production that will be most enjoyed by those unfamiliar with previous approaches by the director and adapter, Trevor Nunn, to Porgy and Bess, the opera, at Glyndebourne, Covent Garden and (in 1993) on film. Those of us doomed to play compare-and- contrast will clock that all the notes are there (more or less), and that the score remains a marvel, while feeling as if one is getting a sort of annotated original rather than the glorious, seismically moving whole of yesteryear.

That this production isn’t more affecting may owe something to the reimagining of an opera to a musical version. But shorter isn’t always better, particularly when handling such ripe, richly outsized music — a score that would seem to demand the form of opera in order to accommodate it. The songs are there, this time with a determined exuberance one associates more with the likes of Five Guys Named Moe. But the story that underpins the music has lost its moorings, requiring such narrative as remains to be taken on faith.

Take, for instance, the union of the kind-hearted, if solitary, crippled beggar, Porgy, with the wayward Bess, the “liquor-guzzlin’” magnet for the likes of her brutish lover, Crown, and the reptilian cocaine-dealer Sportin’ Life. Barely have the two characters met before Porgy is singing Bess, You Is My Woman, her own struggle to live “decent” not given voice or shape until well into the second act. It’s difficult, too, to get a coherent sense of this Porgy, a man who seems less the Peter Grimesian loner of Nunn’s Glyndebourne version, a man transformed by love, than a genial guy tossed this way and that by the demands of a foreshortened plot.

And whereas the second-act storm in the opera always reads (and sounds) like an elemental version of the epic passions that have been unleashed, here it reveals the melodramatic bare bones of a narrative that finds undeniable echoes in our modern, post-Katrina age.

Freshly orchestrated by Don Sebesky, the score retains its power in the hands of the musical supervisor, Gareth Valentine: the choral work on Leavin’ for the Promise’ Lan’ is gorgeous, as is the hymnal quality to the beleaguered community of Catfish Row — a black underclass as imagined by two New York Jews (and DuBose and Dorothy Heyward).

Porgy and Bess itself ought to leave us reeling, especially since few come more marinated in the material than Nunn. But the performers can’t always compensate for what the telling leaves emotionally incomplete, with the men, surprisingly, faring better than the women. Among a 40-strong ensemble that sometimes has to struggle to hold focus on John Gunter’s elaborately shabby set, Edward Baruwa makes something immediate and vivid of the fisherman Jake and his work song, It Take a Long Pull to Get There. Cornell S John’s Crown has a bruising charisma, while O-T Fagbenle’s Sportin’ Life all but knocks There’s a Boat Dat’s Leavin’ Soon for New York clear across to the Big Apple.

If Clarke Peters and Nicola Hughes aren’t the Porgy or Bess of one’s dreams, the growly-voiced Peters does allow glimpses of that great Gershwin romantic who ends the show “on my way” to his beloved Bess. Aficionados may have doubts, but first-timers to the piece will be happy to join him.

The Gershwins’ Porgy and Bess, Savoy Theatre, London WC2, until March 2007. Box office: 0870 1648787