Cosi Fan Tutte, Barbican, London

The touchstone for any director of Mozart's final collaboration with Lorenzo da Ponte is the end. This is the opera about a gruesome experiment on two sisters' fidelity by their fiancés, who dress up for a bet and try to seduce them. The women decide they prefer each other's, but convention wants them back under the male thumb. A hasty papering over the cracks, so to speak, is what the 18th Century would have seen on stage. You can't seriously do that now. What, though?

Some productions go for unhappy ever after; others allow a straight swap. Samuel West, directing opera for the first time, comes up with something more subtle in his English National Opera production. Ferrando and Fiordiligi, tenor and soprano and bearers of the most passionate music, stay the way they have become. The others don't. Dorabella sees Guglielmo outed as a misogynist and yet stands by him, but he backs off. The servant Despina, eager to help the sisters but deceived over the men's identity, is disgusted. Alfonso, who set up the bet, is reviled by everybody. It's a wonderfully Shakespearean solution, true feelings set free by disguise, and the beauty is that it's all there in the text - no secret, but somebody had to make it work.

It explodes out of a deceptively light-touch production which carefully sows the seeds of chaos. Alfonso, though astute and suave, is a dreadful sexist: he greets his male friends with a hug and makes the sisters wait for extravagant hand-kisses. Despina gets a slap on the bum. The three women's scenes are the heart of the action, a dynamo of escalating frankness in which the prime mover is the servant. This is Da Ponte's trademark view of enlightenment emanating from the lower classes, well known in The Marriage of Figaro but rarely brought out so clearly in Cosi.

In achieving its economy of means, the staging, in modern rather than period Naples, stays inside Alison Chitty's box of three full-height panels, on to which Peter Mumford's lighting plot projects mood-enhancing colours. Two sun-loungers are joined by mirrors indoors, shrubs outdoors, and a strip of pebbles. When the men leave for their supposed call-up, they just step without fuss into the orchestra pit, and they are gone - as simple and as shocking as the event itself, perfect preparation for the sublime, numbed trio that follows.

Not everything is so well judged. As she falls for the disguised Ferrando, Fiordiligi peels off his moustache, a deed which ought to bring the whole set-up to an early end - if it's a suggestion of complicity then half of what follows makes no sense. In the final seconds another foursome suddenly arrives downstage, presumably the next set of victims, a nice thought but it fatally divides the audience's attention when the main events are already so unusual.

Fiordiligi and Ferrando, Mary Plazas and Colin Lee, were outstanding on the first night, but the whole six-hander (done, ironically, without ENO's carefully preserved chorus) is strong. The orchestra, conducted by Mark Wigglesworth, is fast and light and as sensuous as the acoustic dead zone of the Barbican pit allows.

To 11 October (020-7632 8300)