The Producers, Theatre Royal, Drury Lane, London

If you thought that the movie of The Producers put the "kamp" in Mein Kampf, just wait till you see this stage-musical version. Having goose-stepped its way into the Broadway record books with its 12 Tonys, its $1m- a-week box-office takings and the astronomical price of its black-market tickets, Mel Brooks's glorious show now hits the boards of the Drury Lane Theatre, in London, "mit a bang, mit a boom, mit a bing-bang, bing-bang boom". To say that it unleashes an epidemic of bliss would be too mealy-mouthed.

In some ways, this is peculiar material to want to turn into a musical. On screen, Springtime for Hitler - the transcendent stinker that is supposed to be a fortune-making, certified flop - stands out as a stage-show-within-a-film. In a theatrical extravaganza, where everything is a production number, it's hard to keep the deranged values of this crackpot climax distinct from the loopy values governing the entire show. And, on screen, Bialystock and Bloom are unscrupulous con men who want to make a dishonest buck, not also appealing song-and-dance men with whose stage-struck fantasies we can identify.

Hard-core fans of the film may well feel that the change of medium involves a loss of bite. The compensations, though, are considerable. Even in the movie, Brooks was clearly more than half-besotted with the showbiz ethos he mocks. In the stage version, his mad, mischievous adoration of the musical genre knows no restraint. Aided by the terrific zip of Susan Stroman's direction and choreography, The Producers comes across as an insane love letter to old Broadway and to the classic shows and films about putting on a show.

It's delivered by the brilliant partnership of Nathan Lane (who gamely replaced Richard Dreyfuss) and Lee Evans. As the producer Bialystock, Lane is a roly-poly bundle of delicious disreputability. Whether mugging cardiac arrest all over the furniture at the mere thought of investing his own money in a production, or summarising the whole show in a hurtling five-minute reprise of consummate showmanship, he's so at ease in the role he seems to be performing it and offering an outrageous descant on it at the same time. Playing Bloom, the uptight accountant, Evans is adorable both in repressed-hysteric mode and in the goofy, enchanted innocence with which he takes to the unfamiliar world of theatrical magic.

The evening unfolds as a sublimely silly festival of awful gags and musical in-jokes. For example, the role of Hitler is assumed, at the last minute, by Conleth Hill's splendid Roger De Bris, the cross-dressing worst director in the universe. There's a naughty nod to 42nd Street when his poisonous assistant (James Dreyfus) encourages him with the deathless line: "You're going out there a silly hysterical queen and you're coming back a great big passing-for-straight Broadway star!'

Likewise, the "Loveland" section in Follies is cheekily alluded to in "Little-Old-Lady-Land", in which Bialystock's sex-mad elderly backers break into an idiotic fantasy dance routine with their Zimmer frames. And where else but in this concoction would the Führer have a Judy Garland sequence? The show comes daringly close to confirming the view, satirised in its Springtime for Hitler sequence, that everything is showbiz.

Nathan Lane has to leave the production in early January. The producers of The Producers should plough some of their enormous profits into trying to clone him.

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