Anaïs Mitchell's 'Hadestown', Union Chapel, London

At last, a musical without the jazz hands, and a folk opera made in heaven

There are risks to be run in any profession, and while it is of no great significance in the grand scheme of things, finding out about the best album of 2010 a few weeks after compiling your end-of-the-year list is about as grave a mistake as it is possible to make if you review music for a living.

In my defence, it does not sound promising on paper. Hadestown is the "folk opera" retelling of a Greek myth, setting the story of the doomed lovers Orpheus and Eurydice in a "post-apocalyptic Depression-era" America. It is written by the Vermont-based folkie Anaïs Mitchell, whose Joanna Newsom-esque voice might politely be said to divide listeners.

That the Hadestown album is such an unmitigated triumph (five stars across the board, the highest ever rating at review aggregator website AnyDecentMusic?) is due to Mitchell's immaculate songcraft, the orchestral arrangements of Michael Chorney and the inspired casting of Justin (Bon Iver) Vernon as Orpheus.

Less folk opera and more a celebration of the Americana idiom from jazz to vaudeville to folk to blues to Broadway, listening to Hadestown is to imagine a musical in your mind unencumbered by any of the scenery shifting and jazz hands that might mar its production in the real world.

Which brings us to a sold-out Union Chapel in London where anticipation and tealights are flickering in unison. As with each live performance of Hadestown, tonight the various roles will be performed by a hastily assembled cast of the venerated and the available, so Jim Moray has the unenviable task of trying to replace Vernon, Martin Carthy will play Hades, Thea Gilmore will step up to the plate as Persephone and so on. It's a ramshackle idea perfectly in keeping with the nature of the project, though, as things turn out, it is not without its disappointments.

For a start, the acoustics of the venue make the subtle percussive moments of the score sound like a scene from Stomp. Moray struggles to find his falsetto and Carthy is more sweet uncle than terrifying king of the underworld. And then there is the troubling aspect of the narrator: for this occasion the Scottish folk singer Jackie Leven. Breaking off every few songs to tell everyone present a story they already know from the CD is an unnecessary distraction that serves only to break any spell the music creates.

And yet, in spite of all this, there are moments where the sheer brilliance of the music overpowers all the obstacles in its way: Mitchell plays her part of Eurydice to perfection; Gilmore oozes a Jessica Rabbit-like sensuality missing from Ani DiFranco's recorded reading; the Fates (Wallis Bird, Nuala Kennedy and Sharon Lewis) sound as if they were indeed fated to sing together; and the chorus numbers have enough pizzazz to turn this austere setting into a sleazy speakeasy.

At the show's centre is a song so sensational that even the discomfort of our hardwood pews is forgotten. "Why We Build the Wall" is both the story of life in the mythical underworld and as potent a parable as it is possible to write. "Why do we build the wall?/We build the wall to keep us free/And the wall keeps out our enemy/What do we have that they should want?/We have a wall to work upon/We have work and they have none/That's why we build the wall."

It's a piece of musical theatre that in itself deserves the ovation on which the night ends. Though, as with all professions, it seems recreating a note-perfect folk opera from scratch each night is not without its risks.