Ondine, Royal Opera House, London WC2

It came from under the waves

Approaching the Royal Opera House from the lower end of Bow Street, raise your gaze to second-floor level and you meet with a puzzling sight: a 20ft-high photograph of a gauzily clad ballerina diving to the bottom of a swimming pool. The uncaptioned image has been there since 2001, the dancer (Sarah Wildor) quit the Royal Ballet soon after, and the work she was promoting - Frederick Ashton's Ondine - has been off the menu all that time. Yet there the thing stays, impossible to remove owing to the cost of hiring a crane and blocking the street.

Approaching the Royal Opera House from the lower end of Bow Street, raise your gaze to second-floor level and you meet with a puzzling sight: a 20ft-high photograph of a gauzily clad ballerina diving to the bottom of a swimming pool. The uncaptioned image has been there since 2001, the dancer (Sarah Wildor) quit the Royal Ballet soon after, and the work she was promoting - Frederick Ashton's Ondine - has been off the menu all that time. Yet there the thing stays, impossible to remove owing to the cost of hiring a crane and blocking the street.

For the next four weeks, though, the poster has a point. Ashton's last full-length ballet, created in 1958 as a vehicle for Margot Fonteyn, is back for nine performances in all its fishy magnificence. The story of Ondine, the water sprite who falls for a human with tragic results, inspired Hans Christian Andersen's Little Mermaid and Dvorak's opera Rusalka, as well as a clutch of Romantic ballets. Ashton's bold stroke was to commission a score from Hans Werner Henze - shock horror, no tunes! - plus designs so sophisticated as to show a life-size galleon leaving its berth and a full-scale shipwreck.

The practical challenges, conceived in a cheerfully low-tech Fifties manner, may be one reason why the ballet is rarely done. More likely, though, is the fear that today's audiences won't be sufficiently interested in the plight of yet another deceiving aristo whose silly heart is broken. And up to a point, that is true.

The joy of this revival, led by Tamara Rojo and Jonathan Cope, and including a stunning debut by Ricardo Cervera, is that it doesn't ask for personal identification with its characters, and they make no attempt to be real. Instead, in Rojo's Ondine, we're invited to marvel at the non-human qualities a body can acquire. Where Fonteyn responded to the role as "naive, shy, loyal and loving", Rojo finds a cold, feral inhumanity that's infinitely more intriguing. From the moment Ondine steps from her fountain, Rojo makes us aware of her every amphibian sensation: the rush of air on wet skin, the flick of water from fingers and toes, the touch of tickling leaves or cool stone. When she moves she's a sliver of mercury, so quick and fluid that you scarcely credit this body with flesh and bone.

These effects are only made possible, of course, by Ashton's choreography, which overflows with inspired identifications. Each time Jonathan Cope's elegant Palemon hoists Ondine into the air, her feet tremble like little flippers. When he pursues her to her underwater lair, she creates with her arms an imaginary carapace of shell. The same sea-life imagery, writ large, translates thrillingly to the corps, who as fellow mermaids sway like fronds of algae or stream along invisible currents.

But Henze's glittering music is the dominating force. I grant it's a difficult score to dance to, with the pulse well hidden within its general sheen, but it's fabulously atmospheric and often rampantly exciting, bringing the close of Act I and its crazy sea-bed cop-chase to one humdinger of a climax. It's frankly hard to accept that at the beginning of the 21st century ballet audiences can be turned off by a lack of something to hum on the way out.

Fifty years bring a sharpened perspective on everything: Lila de Nobili's painterly gothic sets (newly restored and ravishing), the imaginative eye that sees a tempest in the flapping of tattered silk scarves, and not least the curious hunting scene. When Ashton brings on the horsewhips we now know it's with a wink and a giggle.

jenny.gilbert@independent.co.uk

'Ondine': Royal Opera House, London WC2 (020 7304 4000), in rep until 24 May