The basic premise of this production is completely bonkers. The first warning comes during the Overture: as the cast freezes, an invisible voice tells us about Ariodante’s state of mind, concluding (as subtitled) ‘he stopped in front of the unknown. She was a woman.’ At the start of Act 3, the voice proclaims ‘The Truth! We have no choice left but confess he was a woman … She was man; she was woman.’ By then, Ariodante has changed from stout hero to a figure in a décolleté black dress, still with a full set of whiskers, for all the world as though he had turned into Baba the Turk in The Rake’s Progress. And the transformation continues: at the end of the opera Ariodante is all woman, quite clean-shaven.

If you think you can stomach this nonsense, an absolutely brilliant performance awaits. Ariodante opened at the new theatre in Covent Garden on January 8, 1735. The soprano castrato Giovanni Carestini took the title-role, and the cast included Anna Maria Strada del Pò and Cecilia Young, the wife of Thomas Arne. Handel’s favourite tenor John Beard played Lurcanio. The composer made use of Covent Garden’s troupe of dancers led by Marie Sallé by writing ballets for each act. Here the set designer Johannes Leiacker has picked up on this French connection, with backdrops in the manner of Poussin or Claude adding colour to the otherwise plain set of walls and doors. Ursula Renzenbrink’s costumes range from slacks and shirtsleeves to full-blown baroque.

The plot, from Ariosto’s epic poem Orlando furioso, turns on Polinesso prevailing on the naive Dalinda to meet him at night disguised as Ginevra, Ariodante’s betrothed, whom Polinesso wants for himself. On witnessing this, the distraught Ariodante disappears and is thought dead; Polinesso defends Ginevra, on trial for her life, but is slain by Lurcanio in revenge for his brother’s supposed death. Ariodante returns and all is explained: he is reunited with Ginevra and Dalinda gives in to the persistent Lurcanio.

If Handel had had a cast half as brilliant as this he would have been delighted. Christof Loy has clearly thought hard about the characterisation and he has been rewarded by his singing actors with performances of deep emotion, tenderness, subtlety and – sometimes – extravagance. To take one example of his attention to detail: when, in Act 1, Lurcanio is pleading in the disorienting hemiolas of ‘Del mio sol vezzosi rai’, Dalinda shows through gesture and facial expression that she is moved. That this is alien to the anonymous librettist’s intention is clear from the succeeding aria, ‘Il primo ardor’, where Dalinda reaffirms her obsession with Polinesso: nonetheless, it is psychologically acute and it prepares the audience for her accepting Lurcanio at the end.

The opera opens with a sarabande, where Ginevra – with necklace and mirror – artlessly sings of her joy. After a duet in thirds with Ariodante she is almost a schoolgirl in ‘Volate, amori’, screaming happily in the ritornellos. The American soprano Kathryn Lewek makes an astonishing transition from these expressions of innocent happiness to the distraught figure suffering nightmarish visions at the end of Act 2. This scene was dropped by Handel, but it is powerfully effective. Loy turns it on its head, though, and Ginevra’s brief accompanied recitative is assigned to Ariodante.

It’s a bit perverse to cast a countertenor as Polinesso, a role written for Maria Caterina Negri (not that Handel would have minded); but Christophe Dumaux is wonderful. He looks a perfect villain with his designer stubble, and he is horribly believable in his scenes with Dalinda, from the smoothness of his pretended wooing in ‘Spero per voi’ to the violence of ‘Se l’inganno’. Sandrine Piau gives a virtuoso performance, whether she is being flirtatious, deceitful or angry. Dalinda is reconciled with Lurcanio in a gentle duet; but he keeps questioning her about her love for Polinesso, which presumably explains why they look rather grim at the end. Rolando Villazón, not really in his comfort zone, makes a credible lover as well as a doughty defender. As the King of Scotland, Ginevra’s father, Nathan Berg is given the beautiful siciliano ‘Invida sorte’, another piece that Handel dropped. With his rock-solid bass voice, Berg presents a dignified figure, torn between his love as a father and his duty as a monarch.

All these characters are given reams of semiquavers to dispatch, usually at high speed. The singers are so good that it would be invidious to single one out, though an exception must be made for Cecilia Bartoli, whose coloratura from ‘Con l’ali di constanza’ to ‘Dopo notte’ will take your breath away, especially when she affects to smoke a cigar during the da capo of the latter. But coloratura is only the half of it: her slow entrance aria, with added birdsong, is exquisitely phrased, as is the famous lament ‘Scherza infida’. Given the weird conceit of Loy’s production, Bartoli’s acting is as accomplished as you could reasonably expect.

In the pit, the period-instrument Les Musiciens du Prince – Monaco play with verve (excellent horns and recorders, and there’s even a musette in the second Musette). The pitch is normally a semitone below today’s standard; Bartoli’s arias dot around between a semitone below and a tone above. The opera is given almost complete: a couple of lines of secco recitative are cut, the Rondeau at the end is omitted and there are savage cuts in the final duet for Ginevra and Ariodante. The evening in the theatre must have lasted over four hours: the conductor Gianluca Capuano is speedy for most of the time but occasionally – as in Ginevra’s ‘Io ti bacio’ – you wish he would get a move on. Barmy concept; compulsively watchable.