Theatre Martin Guerre Prince Edward, London

"Tell me, who are the impostors here?" sings the priest in the new Boublil / Schoenberg musical.

The main impostor on stage is, in fact, the show itself. It calls itself Martin Guerre but it has no real right to the name: everything that has made that story subtle and ambiguous in previous treatments has been removed and replaced with vacuous melodrama.

The Depardieu movie kept you guessing about this man who returns to his 16th-century French village after many years' absence claiming to be Guerre and it left open, until the end, the question of how far his wife knew that Mr Right, so to speak, was the wrong man. The rich conundrums of identity, of recognition, of whether we can lay claim to what is not ours if we do it good are blithely jettisoned here. You can understand why: as Auden wrote, "music cannot exist in an atmosphere of uncertainty". Equivocation is decidedly not the forte of big-budget musicals and once the adapters of Guerre realised this, they should have dropped the project.

Instead, they have disembowelled the story and set it within the completely phoney-seeming context of the wars of religion and the persecution of Protestants. Exactly who is who is made laboriously clear from the outset. In place of a wife who ironically learns the meaning of love in the arms of a counterfeit spouse, we're given Juliette Caton's pretty, cardboard heroine who innocently recognises that this Guerre is a stranger but falls in love with him at first sight anyway, even though five minutes beforehand she had been so adamant about remaining faithful to her absconded, uncaring husband and so unhappy at the Catholic church's attempts to remarry her that she'd joined a secret sect of Protestants.

That's the level of plausibility. The couple's love, their born-again Protestantism, the religious intolerance that threatens to tear them apart are all tackled with a spectacular shallowness. "All I know is that all I want to be is to be close to you," they warble in one of the more moving songs from a score that betrays its influences so often (a touch of Grieg here, a bit of Khachaturian there), it's like a through-sung IOU. The anthemic numbers for the chorus of villagers are reminiscent of Orff on an orff day, while Edward Hardy's lyrics ("His arm is neither too long or too short/ and his feet are exactly the size that they ought") make you wonder what the lyrics of the lyricist who was dropped could have been like.

Choreographer Bob Avian has come up with a stamping dance-step for the suspiciously well-groomed peasants. This injects some much-needed life into the proceedings, though it puts you in mind of tap-dancing for the slow-witted. Nick Ormerod's simple design of revolving multi-purpose, wood-framed towers allows Declan Donnellan's production to achieve an impressive fluidity but all their Cheek by Jowl talents can't disguise the fact that there is an enormous emptiness at the heart of this product and that "product" is the right word for it. With a fine, natural-sounding voice and a strikingly handsome presence, Iain Glen, as the hero, keeps reminding you that there is such a thing as human depth. And that, in these circumstances, is some achievement.