"Life imitating art, imitating life... art imitating life, imitating art." Reflections on this two-way reflection process are delicately delivered in the final song of Enter The Guardsman. This new musical by Craig Bohmler and Marion Adler feels, however, more like a case of art imitating art, imitating art. We just have to be grateful that the art to which it pays homage is very good art indeed.

Adapting a Molnar comedy about an actor who probes the fidelity and the fantasy world of his actress wife by seducing her in disguise, the show adopts the manner of Sondheim's bitter-sweet waltz musical A Little Night Music. It never achieves the astringency of that piece, but Sondheim would not be ashamed of a song like "True To Me" where the actor's joyful relief that his wife's virtue has survived the first disguised assault keeps getting undermined by another voice inside his head which attributes her non-surrender to his poor performance as the guardsman.

I haven't seen the Molnar original but I suspect that Scott Wentworth's adaptation has invented the hapless middle-aged playwright (Nicky Henson) who, attracted to the wife and the husband, is pander and note-taking voyeur to the proceedings. In lovely voice and capturing both the pained yearning and the witty mischievousness of the part, Janie Dee keeps you guessing about whether the unfulfilled wife knew all along that her night of extra marital passion was spent with her husband. Is there a hint of the homoerotic in the husband's heated obsession with the thought that he has a rival? As the playwright, not so tactfully asked, "Who's fantasy is it?" The comically determined virility of Alexander Hanson's well sung actor/guardsman allows that question to linger lightly without in any way sabotaging the realistic rapprochement.

Jeremy Sams's production has camp fun with the theatrical setting - the wheeled in his and her's dressing rooms for those Kiss Me Kate-style backstage bickerings and the obligatory gay technical staff. But, particularly in the hauntingly orchestrated partner-swapping waltz at the close, Sams also brings out the metaphysical mystery. Where do art and life, role play and reality begin and end? "I read it in a book/and it broke my heart/art imitating life imitating art," sings the playwright.

The show is sponsored by Andrew Lloyd-Webber's Really Useful Group and offers a piquant contrast to The Fix, developed under the aegis of Cameron Macintosh, premiered at the same address earlier this year. Taking on the sleazy corruption of American politics, The Fix was certainly adventurous but it demonstrated the primary of slick packaging over content. Enter The Guardsman plays a safer game and is derivative, but it has depth and is much better scaled to the intimacy of this venue.

Donmar Warehouse (0171-369 1732). To 18 October