OPERA Platee Edinburgh Festival Theatre

Jean-Philippe Rameau was never sure what sort of piece his Platee was. An opera? A ballet? A comedy? The same uncertainty seems to have spread to the Royal Opera in its new staging for the Edinburgh Festival. It's true that the work is full of divertissements; you could not just rely on the opera chorus to stand in as part-time dancers. But handing the whole production over to a choreographer, Mark Morris, has produced a new sort of performance that would have puzzled Rameau exceedingly.

The chorus - the ones who sang, that is - were in the orchestra pit. When they were singing, we saw troupes of dancers, some dressed as animals, mostly in colourful leotards. Since choral passages were sometimes very short, this meant that little clutches of dancing figures often swept in from the wings, only to sweep out again at once.

This was not Morris's only contribution. He was also producer and, along with his designer Adrienne Lobel, had decided to cast the piece as a slapstick farce. It began in a New York bar, with Bacchus as barman and Thespis (Mark Padmore, also later Mercure) as a squalid drunk. There were toffs in evening dress, babies in nappies and a pantomime horse; Cupid had an arrow through his head instead of in his hand. The whole thing resembled Offenbach. Even the dancing, witty and wanton though it was, seemed eventually rather routine.

This kind of thing is fine for 10 minutes or so. But the turning of Platee into a frog (a triumph of make-up) was sure to pall. Jean-Paul Fouchecourt had a lot of singing to do, which he did with delightfully light, elegant tone and much mock pathos; but, because of his long periods on stage, his constant flapping around on huge green feet, coupled with a grotesque face incapable of conveying visible emotion, eventually became tiresome. It was, of course, meant to be politically correct: you cannot laugh at a fellow-human nowadays for being ugly.

There was much to enjoy on the musical side, however. Nicholas McGegan had the superlative Royal Opera House Orchestra playing with the kind of vitality that makes authentic instruments unnecessary. Padmore sang Mercure with an almost depersonalised, spiritual tone, but there were plenty of real operatic voices too: Susan Gritton (as Thalie, later Clarine) conveyed excitement and brilliance, and the fabulous Nicole Tibbels performed Folie's set-piece - the Queen of Night on ecstasy - with glittering elan. In Act 3 there was at last a voice of impressive proportions: Diana Montague as Junon, commanding, lustrous, awesome.

Platee is a marvellous piece, written with a bravura that sometimes sounds almost like modern pastiche. Can the sly, perverse entr'acte between the prologue and Act 1 really have been written by a baroque composer? It is also full of satire and pathos: even ugly, stupid people deserve a bit of our time, it seems to say. It should touch the heart a bit, but it's hard to commiserate with a frog.

7.15pm tonight, tomorrow, Edinburgh Festival Theatre (0131-473 2000) and from 22 Sept at the Barbican, London (0171-638 8891)