The Philosopher's Stone, Garsington Opera, Oxon

The Philosopher's Stone premiered at Emanuel Schikaneder's Theater auf der Wieden in Vienna in 1790, just one year before The Magic Flute. Unlike Flute, though, it was a committee job, with Schikaneder himself joining the composers Johann Baptist Henneberg, Benedikt Schack, Franz Xaver Gerl and Mozart to pull in big audiences to Vienna's "people's theatre".

The Philosopher's Stone was one of a series of fairy-tale singspiels (book-and-song musicals) commissioned to turn around the theatre's fortunes, and speed was of the essence - hence the split creative team; hence the intrigue. Had Mozart merely made the tea, we'd be gauging his influence. In the event, his contributions were probably made while the tea was brewing.

Now Garsington's conductor, Steuart Bedford, and director, John Cox, have added to the intrigue by niftily fleshing out what is actually a pretty rum piece with some cleverly chosen bits of lesser-known Mozart. He is suddenly slightly more of a presence in the score. And he's always very conspicuous.

I deliberately tried not to read the complicated breakdown of attributions before taking my seat, but as soon as the heroine of the piece, Nadine (the fresh-voiced Amy Freston), began her establishing aria, the character of the woodwind writing alone was an instant give-away. Mozart just can't help catching your ear and confounding your expectations wherever you put him.

But a far more compelling reason for reviving The Philosopher's Stone lies in the uncanny ways in which it foreshadows The Magic Flute. All the characters are there in embryo; aspects of the musical characterisation, too, suggest that Mozart may have been more of an influence than we know. The enchanted flute itself is here the voice of a magic bird. And the opera has its very own Papageno in the jolly gamekeeper Lubano, energetically and engagingly played here by Leigh Melrose. His wife, Lubanara (played by the pretty and charming Teuta Koco), undergoes a rather different transformation from Papagena, briefly turning into a friendly feline. That bit we know to be Mozart. And if we don't, it's fun finding out.

The expert Cox and his designer Peter Ruthven Hall find ways of transforming Garsington Manor's back garden into "the enchanted island" of the opera's subtitle. When our protagonists' ship goes down, heads and arms pop up through a billowing blue sheet - not waving but drowning. The benevolent spirit Astromonte's messenger (Katherine Bond) arrives on a golden airborne bicycle. The choreographer Scott Ambler engages his very own dancing clouds. And the forces for good and evil bring us the golden-haired and golden-robed Astromonte himself, whose big aria with coloratura is enough to make even the Queen of the Night blanche, the part expertly taken by Iain Paton; and, as a very hirsute blue genie, it's impossible for Michael Druiett as the malevolent Eutifronte not to make his mark.

The most spectacular show-stopper of the evening (definitely not by Mozart but by Gerl) had poor Ashley Catling, as the hero Nadir, attempting to negotiate a Niagara of notes and no less than four top Ds. Catling's voice seems to be trapped somewhere in his nasal cavities but maybe, just maybe, Mozart would have released it for him. We will never know.

To 10 July (01865 361636)