In Janáček's opera The Makropulos Case, the operagoer must accept a single premise of implausible fantasy: from then on, the action flows inexorably and logically to its conclusion. It's a device used by a great deal of fantasy literature, from the low-brow of The Day of the Triffids to the high-brow of Janáček's compatriot Kafka's Metamorphosis. In The Makropulos Case, the device is deployed with great theatrical power.

SPOILER WARNING: if you don't want to know the plot, don't read on!

The central question of the opera is what would happen if a woman actually found and drank an elixir which made her young and alluring forever (or, in the case of Elina Makropulos, for 300 years). Janáček's answer - protrayed with complete credibility - is that her life and the lives of everyone she touches would be quite dreadful in a whole variety of ways that are thoroughly explored in the opera. All men want to possess her, many want to kill her if they discover they can't. Motherhood ends up as a succession of "thousands of bastards running around the world". If she does find true love, she must inevitably be left to mourn as her lover withers and dies. Even having the same name for that length of time becomes unbearable: by the time we are introduced to Elina, she has become the diva Emilia Marty, having gone through Ellian McGregor, Eugenia Montez and a dozen other aliases (obviously retaining a fondness, however, for the initials "E.M.").

The opera is based on a play by Karel Čapek (also, by the way, one of the early pioneers of science fiction and the creator of the term "robot"). The play is described as a comedy, and there may be plenty of humour around, but the music and the thread of the action tell a different story: if this is humour, it's blacker than black. And Janáček's music is quite unlike anyone else's: without using the atonal harshness of later 20th century composers, it's mercurial and full of invention, continually moving in tone and shape to complement the action.

Last night was the first time I've seen The Makropulos Case, and the ENO production and performances did an excellent job of bringing out the story and the drama. Musically, I was less convinced: the overture sounded confused, and for the first two acts, I kept feeling that I was missing something. There was some level of coherence that seemed absent, although it all improved greatly in the denouement in Act III. Listening to Charles Mackerras's recording as I write this, I can see that this a difficult work to carry off: Janáček throws you helter-skelter into the action with no compromise whatsoever. But overall, there's a glorious delicacy and lightness of touch about the score, and I felt that conductor Sir Richard Armstrong and the ENO's orchestra simply didn't nail it.

I thoroughly enjoyed the production and the acting. Monochrome costumes and sets which used the full depth of a very long stage provided a real feeling of alienation. Amanda Roocroft was commanding as our multi-identitied heroine: although it's a short opera, it must be an exhausting role as she is at the centre of attention on-stage for virtually the whole production. The other roles are subsidiary to this tour-de-force but were ably handled, even if no-one really hit the heights vocally. Ashley Holland was notable as Baron Jaroslav Prus, and Ryland Davies provided a fine cameo as the elderly and batty Hauk-Šendorf.

But I don't want to overplay the criticism. I'm sure it's possible to perform The Makropulos Case better than this performance, but it still blew me away with its powerful portrayal of "beware of what you wish for". It's a particularly appropriate message in these days of obsession with celebrity, youthfulness and cosmetic surgery: this is one production that you can be sure L'Oréal will not be sponsoring. It's a great night at the opera, and warmly recommended.