Before Handel opera got cool, there was John Copley's staging of Semele. There is no chic monochrome design scheme here, no topical political subtext, no similarity intended to actual persons living or dead.

But it looks no more dated now than it did when it was new, 21 years ago. Copley presents this gently witty story of the schemings of mortals and gods by and large as if it were still the 1740s, when the score was written. And he has returned for this revival to direct a largely new and first-rate cast.

In Henry Bardon's sets and David Walker's sumptuous costumes, it looks delectable from the start. An opulent opening procession brings us crinolined ladies, sultans in pumpkin hats, heaving bosoms and fluttering lids aplenty - all before a note is sung. In true 18th-century style, the clouds and trees are conjured up, surprisingly effectively, with painted panels sliding on from the sides and top; Iris makes her entrance from on high, perched below a rainbow. Semele, singing an aria on a cloud, surrounded by cherubs, belongs on a fresco ceiling by Tiepolo.

The comedy takes a while to get going, and the biggest laugh during act one came when Jupiter accidentally stubbed his toe on a cloud. But things lift off in this respect with the appearance of John Relyea's excellent Somnus, a snoring tramp of a god lusting after a nymphet who is a quarter his age. And they continue with every appearance of Stephanie Blythe's Juno, a formidable battle-axe, fabulously sung.

Kurt Streit's Jupiter is initially bashful, later ardent. Robin Blaze's Athamas is less commanding than in English National Opera's Xerxes last year, but it is a promising company debut nonetheless. Yet it is the American soprano Ruth Ann Swenson who rises to the greatest challenges. For a start, she makes an almost likable character of the air-headed Semele, and her aria as she flirts with her newly ravishing self in the mirror is the evening's vocal highlight. It is long and dazzlingly difficult, but Swenson sounds positively relaxed. In the pit, Charles Mackerras does not always draw maximum beauty from the orchestra, but does create a constant and responsive rapport between the orchestra and the stage.

This Semele might give you a strange impression of the modern world of opera; but on its own terms it is a gem.

· In rep until July 11. Box office: 020-7304 4000.