Richard Strauss's Ariadne auf Naxos boasts the most confusing cast list of any opera. Ariadne and Bacchus feature either as themselves or as "primadonna" and "tenor", in addition to figures from Greek mythology, we get Commedia dell'arte names like Scaramuccio and Truffaldino, there are backstage characters like the composer, the music master and the wigmaker, and a spoken part of the Major-Domo. And that's without mentioning the most important person in the opera: the unnamed "Richest man in Vienna", who is giving the party and paying the performers, is in control of everything that's going on, but we never see or hear him in person.

All this comes from a strange genesis: the opera started out in 1912 as incidental music and a postlude for a translation of Molière's "comédie-ballet" Le Bourgeois Gentilhomme, a bitingly sarcastic comedy of manners which derides not only the vulgar and stupid Monsieur Jourdain (who aspires to a gentility that his birth makes impossible) but also the rapaciousness of the teachers he hires and the fripperies of the nobility. It didn't go well: Strauss's postlude ended up taking 90 minutes, making a total evening of over six hours, and even ardent Strauss fans didn't like it.

In response, Strauss created a new version in 1916, replacing the performance of Le Bourgeois Gentilhomme with a new "Prologue" incorporating some of Molière's characters (the Dancing Master, the Music Master, the Composer, etc). The basic concept is this: the main opera seria is a contrast between high art and the burlesque of Commedia dell'Arte, the high art being provided by the story of Ariadne abandoned by Theseus but redeemed by the love of Bacchus, with the burlesque provided by the gorgeous Zerbinetta and her troupe. The prologue builds a backstage story as to how these two improbable bedfellows came to be fused together in the first place: the two companies have been forced to work with each other at the last minute command of the "richest man in Vienna", who is footing the bill and therefore has absolute power.

I thought Welsh National Opera's rendering of the prologue was quite fabulous. The joy of the work is the comic interaction between the pretentiousness of the Music Master and Composer (who have visions of providing the high art and pathos of Greek tragedy enhanced by their music) set against the coarseness of the Dancing Master and Zerbinetta (who worry about whether they will be able to wake up the audience after they've slept through the opera seria) with the ring being policed by the Major-Domo, who is simply a conduit for his master's uncultured whims. The dialogue is very funny, and the WNO cast brought out every nuance of it, while conductor Lothar Koenigs attacked Strauss's deliciously lush score with gusto. Robert Poulton was a splendidly funny Music Master, Eric Roberts also hilarious in the spoken part of the Major-Domo. Imelda Drumm sang well and did a wonderful job of "tortured aesthete" as the Composer, but wasn't persuasive when being seduced by Gillian Keith's Zerbinetta: no doubt it's difficult to summon up the required sexual chemistry when in a trouser role, but it's required in order to give that part its full effect.

After the interval, we get the main opera seria, which has the lushest and most engaging of Strauss's music and provides the main ability to shine vocally for Zerbinetta, Ariadne and Bacchus. Gillian Keith sung beautifully, negotiating both lyrical passages and some fearsome coloratura, while her acting was outstanding, presenting the ambiguity of the character (is she a vulgar starlet, or is that just a stage persona which hides the real Zerbinetta underneath?). As Bacchus and Ariadne, Ricardo Tamura and Orla Boylan produced a lovely neo-classical sound, while Boylan's acting portrayed neatly the difficulties of interacting with the Commedia dell'arte characters in their robust attempts to dilute the high art with a bit of farce.

However, I'm afraid I found that the opera seria dragged on for much too long. For me, most of the acute and interesting commentary about the nature of high versus low art - and most of the fun to go with - had already happened in the prologue. The main opera has some truly lovely music - the overture itself is pretty rapturous - but I didn't think there was nearly enough dramatic material to justify a length of over 90 minutes and I kept wishing that Bacchus and Ariadne would just get on with it and draw the tale to its inevitable conclusion. This may have been partly down to the pace of the conducting - some performances on CD clock in at around 80 minutes, rather quicker than last night.

None the less, a fascinating trip to see a unique work, and Welsh National Opera gave a performance of very high quality in just about every department.