During the so-called Age of Reason, Samuel Johnson disparaged opera as "exotic and irrational". Johnson might have been referring to the series of operas Handel wrote for London's competing theatres between 1711 and 1741 - exotic farragos set in imaginary kingdoms ruled by star-gazing magi or alluring sorceresses, often concerned with the irrational antics of renegade crusaders or lethally capricious despots. In Orlando, the fabled hero of Ariosto's romance is driven mad by jealousy; in Serse (Xerxes), the Persian monarch falls melodiously but unrequitedly in love with a plane tree, and in his spare time constructs a bridge over the Euphrates which, surrendering to the agitation of nature, promptly collapses. Bad performances of Handel operas show us galleries of statues serenely vocalising. The truth is less poised and noble: Handel's people seem like refugees from Bedlam, the asylum in which the 18th century locked up its rhapsodists.
The Royal Opera's recent production of Orlando, ingeniously staged by Francisco Negrin, locates its hero (the wired counter-tenor Bejun Mehta) inside a padded cell that is the projection of his own inescapable fantasy. Here, punctured by Cupid's arrows while Mars tries to rouse him to deeds of valour, he reels through a deranging dream. The wizard Zoroastro, gruffly sung by Jonathan Lemalu, scrutinises the sky through his telescope and wonders whether any significance can be glimpsed in its scintillating chaos; like Handel looking down on human folly, he shakes his head over men who worship a blind god. Meanwhile Anthony Baker's set crazily revolves, and having sampled a flurried succession of environments - a craggily sublime romantic landscape, a pastoral idyll furnished with stuffed sheep, a formal and stifling country house - it falls apart into a toppling, disoriented wreckage. The demented Orlando rails in a mirrored room where the names of the characters he obsessively pursues are scrawled on the glass.
Finally, after his merciful loss of consciousness, he and all the other characters sink to the floor in exhaustion and fall asleep, soothed beneath the counterpane of a theatre curtain. Their shared coma, exquisitely played by the Orchestra of the Age of Enlightenment and its conductor, Harry Bicket, represents what Goya called the sleep of reason, which has been narcotically overpowered by desire. The orchestra should change its name: Handel's music is hardly enlightened - like Zoroastro's spyglass it peers into the dark gulf we carry around inside our heads.
At the Theatre des Champs-Elysees in Paris, William Christie and Les Arts Florissants have settled for a more frivolous and superficial view of Serse. The Persian setting gives the director Gilbert Deflo the excuse for some flippant exoticisms: a harem girl puffs on a hookah in the slow section of her aria and then, needing to increase her neural tempo when the fast section arrives, swigs a cup of coffee to accelerate her pulse, while the surly, discontented Serse lounges on an outsize version of the peacock throne. These oriental excursions combine uneasily with Deflo's homage to the Neapolitan commedia dell'arte in the opera's comic passages. The army here consists of toy soldiers with twitching, lopsided moustaches glued into place; the production misses the threat of violence in a society dominated by a monarch who sings a grateful homily to a pot plant and then orders the palace guard to slaughter his brother.
The role of Serse was composed for one of the preening castrati who commandeered the operatic stage in Handel's time; in Paris, it is sung by the astoundingly versatile Swedish mezzo-soprano Anne Sofie von Otter, who presents the king as a petulant monster with a whim of iron. Imagine, if you can, a tall, thin, haughtily elegant woman playing Don Giovanni. Otter is dressed as a saturnine prince, her lacquered black hair tugged into a spiky pigtail, with a blood-red sash tied around her waist. She scuttles on a spider's long, swift legs, wraps serpentine arms around her prey, and does a courtship dance like a randy pigeon. Every sense is polymorphously let out to play: at one point she sniffs the shoes of the princess Serse dotes on. The character's libertinism determines the free-spirited way in which Otter sings - jazzily elasticising the rhythm whenever Handel repeats himself, ranging dynamically from lovelorn whispers to a volcanic outburst when Serse irritably demands that the sun eclipse itself and orders the apocalypse.
Serse experiments with emotions and rapidly discards them. Each of his brief, spasmodic arias gives vent to a different mood. He is rabidly angry, skittish and grief-stricken by turns, saved from the delirium of Orlando only by an awareness that these effusions are no more than vainglorious display, performed for his own benefit. Here, brilliantly performed by von Otter, is the embodiment of the hedonistic 18th century, which - despite its reputation for rationality - turned the pursuit of happiness into a reckless creed: Serse is caprice personified.
Serse is performed at the Barbican Hall, London EC2 (020 7638 8891) on 28 November, and has been recorded by Virgin Classics