The Enchanted Island: the isle is full of mash-ups

I've just written a new opera that premiered at the Met in New York. The beauty of that slightly outrageous statement is that it is not entirely untrue. OK, nothing about my new opera is really new. On the contrary, much of the music is over 300 years old. The characters are even older. The story is fashioned from the most familiar of components. And yet all those notes and all those words in that order, have never been heard before on any stage. So, on a technicality, I'll stand by my opening statement.

Let me explain. This all began about five years ago. Peter Gelb, who had not long taken over at the Met, came to me with a rather bizarre notion. "How would it be," he asked, "if you took hidden gems from the baroque period and turned them into an opera? Oh, and it has to be in English." That was the totality of the brief. To showcase wonderful music that is rarely heard on any opera stage. That was the genesis, and like the best of them it culminated in revelations.

And so I started listening. For over a year, using iTunes and YouTube (the latter an amazing classical music resource – who knew?), I listened to nothing but baroque vocal music. When I say listened, in fact I used the same technique that I deploy in art galleries: the pieces that tap you on the shoulder and demand to be listened to (or looked at) were my first port of call. Music that stopped me in my tracks went on a "must include" list that eventually had well over 24 hours of music.

Handel came first. His Italian operas, over 40 of them, and the oratorios, every bit as theatrical, sometimes more so. The humanist bible that is L'Allegro, il Moderato, ed il Penseroso. The church music: I allowed myself one ridiculously famous example, since the entrance music for the World's Greatest Living Tenor (Placido Domingo) seemed to demand it. And most revelatory, the young Handel of the early Italian cantatas, of masterpieces such as The Triumph of Time and The Resurrection, where we see a fully-formed genius spreading his wings. I listened to everything and, as George Bernard Shaw said of hearing late Beethoven, I did with my ears what I do with my eyes when they stare.

Next came Vivaldi who, thanks to some remarkable recent recordings, is now slap-bang on the operatic map. His "hidden gems" are often pretty expertly concealed. But when you come across them, they grab and galvanise you. Vivaldi has a dark, brooding lyricism and above all a kinetic energy, a virile oomph, unrivalled by any of his contemporaries. He ended up with nine arias in my finished piece, but I could have chosen 20 more.

The other main strand was French baroque. If you think Handel is theatrical, these guys are off the scale. Not just Lully and Charpentier (neither of whom made the cut) but also the weird and wonderful Leclair (who did). The greatest of these, though, is Rameau. Rarely does old music sound so astonishingly modern. As eccentric as Berlioz, and as gifted in writing for dance as Tchaikovsky or Stravinsky, Rameau came to opera even later in his life than Janácek. I had decided to include a masque sequence (more precisely, a "dream ballet"). Its music is mostly by Rameau.

How, though, to make all these various musics feel like one piece? The language was going to help. A new English translation of the works (simple, I hope, and not too flowery) would lend consistency. The orchestra would be useful, too: one band to bind the sound worlds. But it was clear that it would need a story, one that would lead to this music – to cause it, rather than be a symptom of it.

My thinking was that unfamiliar music needs a familiar story. An anti-Mamma Mia!, as it were. A jukebox full of flip sides. And the stories best known to me since my youth (thanks to a scholarly father) were those told by Shakespeare. It was listening to Purcell that first brought The Tempest to mind. Purcell and Dryden's version was called The Enchanted Island, a title too good to ignore for a piece to be premiered in Manhattan. Dryden, though, had spotted that a desert island would, by definition, be slightly low on love interest, and love is what fuels the aria-making machine that is baroque opera. Someone falls in love: love aria. Someone looks on: jealousy aria. Someone is deceived: rage aria.

Dryden's addition of Dorinda, a boy-mad sister to Miranda, wasn't too inspiring. But his fleshing out of the sorceress Sycorax (Caliban's mother and Ariel's previous owner) was too good not to steal. And it wasn't too much of a stretch to imagine Sycorax seduced, spurned and then banished by Prospero to the dark side of the island. He steals her land, her son, her servant, her heart – all handy motives for revenge, and for the revenge aria (we have a cracker in act one, originally from Handel's Teseo), not to mention reconciliation and the forgiveness aria (ditto in act two, from his Partenope).

Still, the lack of love interest remained. The aria machine was demanding more fuel. My second choice of Shakespeare play was A Midsummer Night's Dream, (again because of Purcell, and his Fairy Queen) with its four intertwining lovers. I thought: let's mash the two stories together and see where it takes us. Lysander and Demetrius are maybe accompanying their wives on a honeymoon cruise when they get caught up in Ariel's tempest. Why did he zap the wrong ship? Perhaps because Sycorax corrupted the spell. Maybe she then tries to ensure the future of her island by helping Helena fall for Caliban. Miranda could fall for Demetrius, Lysander and finally Ferdinand – proclaiming "Oh brave new world" to every one of them. Lest things get too complicated, I could hide Hermia in a cave until Act Two. And Prospero could perhaps be forgiven in a Figaro-style finale. It was sort of writing itself.

Meanwhile, the show had to be cast, and quickly. Thankfully, as soon as I thought of The Tempest, I was literally hearing voices. Prospero – numinous, shamanic – should be the counter tenor required in all baroque opera. Mercurial Ariel: coloratura soprano. Caliban: bass. Sycorax: a dramatic mezzo. Miranda: a young lyric soprano. The Met's wish list was remarkably congruent with my needs: David Daniels, Danielle De Niese, Luca Pisaroni, Joyce DiDonato and Lisette Oropesa were free and interested and I was starting to feel like a greedy kid in a sweetshop.

Then came Placido Domingo, whom Gelb had asked to join the project. Could I find a role for him? Could I? You had me at "Pla-". And so the role of Neptune was born. And very handy he proved, if only for plot purposes. A god (and Domingo has, amazingly, never played one) can always alter events and put wrongs to right. I like a character to go on a journey and Neptune is no exception. He starts depressed and irritable – his powers are waning, his oceans polluted – but rediscovers his mojo, as it were, and intervenes in the affairs of men. He brings Act One to its climax with an astonishing Handel scena from Tamerlano.

By now music and story were informing each other. A ravishing sleep aria from Vivaldi's Tito Manlio demanded to be featured, so I wrote a sleep scene. A rage aria from Handel's Hercules followed. Similarly, the performers, now cast, had music they wanted to sing; interestingly, Vivaldi was high on everyone's list. I was only too happy to bend the story to lead to their preferred arias. By this stage, I was contacting our baroque experts, William Christie and Ellen Rosand, and asking questions like: "Is there a fast trio for baritone tenor and soprano, in which the woman is impervious to male wooing?" Yes, in Handel's Susanna. "Is there a mixed quartet that might sound like two couples waking up?" Yes again, in Vivaldi's La Verità in Cimento. "Does anyone know a baroque sextet?" No, so I fashioned one out of an early Handel quartet.

On New Year's Eve, after numerous workshops, rewrites, tweaks and cuts, we opened at the Met. The production, by Phelim McDermott, is sumptuous, and the cast quite simply the finest in the world. As for the piece, well, many New Yorkers have taken it to their hearts. Purists have been suitably and predictably outraged. My only hope is that it should be seen for what it is: a showcase for, and a love letter to, a century of amazing music.

The Enchanted Island screens live from the Met at 80 cinemas around the UK on 21 January. Details: www.metopera.org/uk