The fun starts here

WAGNER was definitely on to something with Tristan and Isolde, as even Giuseppe Verdi had to admit. "This gigantic structure," confessed the staunch Wagner-sceptic late in life, "fills me time and time again with awe, and I still cannot quite comprehend that it was conceived and written by a human being. The second act ... is one of the finest creations that ever issued from a human mind."

Even granted the possibility of irony, these are strong words. Tristan, it seems, stands out among Wagner's works. Even those who resist the Tolkienish gods and giants of Wagner's Ring frequently admit to finding Tristan and Isolde overpowering. Those whom the opera doesn't overpower are obliged to allow, on purely scientific grounds, that Tristan's place in history is a privileged one. Without it, says the text book, modern music might just not have happened. In its second bar, the opera contains the single most discussed moment in European music; even now nobody can quite give an unequivocal technical description of the "Tristan chord".

Wagner wrote to Liszt in 1854, pondering his new project. He had in his head "the simplest but most full-blooded musical conception," he said. "As I have never felt in my life the real bliss of love" - the composer was a self-dramatist before he was ever a dramatist, remember - "I must erect a monument to the most beautiful of all my dreams, in which, from beginning to end, that love shall be thoroughly satiated ... " Alas, Wagner's "highly developed eroticism" was playing up again. Living in Switzerland as the guest of a wealthy silk manufacturer, Otto Wesendonck, Wagner had fallen in love with Wesendonck's wife, Mathilde.

The love affair and the conception of Tristan and Isolde are linked. During the four and a half hours of his "Action", as Wagner called it - it is in fact the most static opera in the repertoire - what is presented on stage is the working of a doomed, illicit love. Between Acts I and II, Isolde marries King Mark of Cornwall, but all along Tristan remains her real love. Almost the entire second act consists of an ecstatic, throbbing love duet between the pair, the most erotic music Wagner ever wrote. The colloquy is broken up by King Mark, who finds Tristan and Isolde lying together on a flowery bank. Finally, in Act III we see Tristan in exile, sick, yearning for Isolde. When she arrives, Tristan dies. Then Isolde dies too. "Profound emotion and grief of the bystanders", as the stage direction says. The couple have achieved their aim, however. They have escaped love's earthly yearning into the higher realm of death: a kind of Nirvana, as the composer's Buddhist readings taught him.

"This Tristan is turning into something frightful!" Wagner wrote to Mathilde. "That last act!!!!" Public reaction in 1865 was also divided. It was too modern, said some, uneventful, dissonant. Scope for acting was limited: all the lovers could do was fall into each other's arms, or not. Subsidiary characters - the King, the lovers' faithful servants - simply disappeared. Tristan and Isolde seemed indifferent to each other as persons: as W H Auden was to put it later, they might almost "have drawn each other's names out of a hat".

The criticism was valid in its own way, but it missed the point. Tristan was a musical and psychological tour de force. The opera's theme was yearning; and in his music Wagner created an extraordinary sonic embodiment of that emotion. The lovers never experience fulfilment on stage; neither, in the pit, does the orchestra: their harmonies move on, gasping, never settling, towards a resolution which is always delayed. When the climax comes, the opera ends. Isolde expires over her lover's body as she sings her orgasmic Liebestod, her song of "love in death". Sexual pleasure, Wagner knew as well as anyone, resides in climax delayed as much as desire satisfied, and in depicting this he took musical language further than ever before.

Tristan is an opera of extremes even in an oeuvre of extremes. But there is plenty for the new listener to grab hold of: the shimmering Prelude itself, with that famous chord (F-B-D#-G#); the ecstatic music of Act II, where even the lovers' identities become confused; Tristan's "delirium" as he waits for Isolde in the final act; Isolde's hymn to death. The action is limited. But Wagner had been convinced by the philosopher Schopenhauer that plot, essentially, was unimportant. What mattered was music, equated with a higher reality, beyond the everyday. In Wagner's own expression, his drama was "a deed of music made visible".

It took some years for the opera's full effect to be felt. "Music reacted to Tristan," wrote Paul Hindemith, "as a human body to an injected serum" - first striving to exclude it, only later accepting it as necessary. Now we speak of music pre- and post-Tristan. Bruckner, Mahler and Richard Strauss all operated in a world of harmony opened up by the opera. But it was the Viennese radicals - in particular Schoenberg and Berg - who really picked up the baton and ran. Schoenberg's early sextet of 1899, Verklarte Nacht, sounded to one listener "as if someone had smeared the score of Tristan while it was still wet". Later, Schoenberg broke down traditional harmony altogether. Berg's Lyric Suite quotes the opera. Even Debussy - great innovator, arch anti-Wagnerian - inadvertently expressed his debt to Wagner by parodying Tristan in The Golliwog's Cakewalk.

All of which makes embarking on a new production of Wagner's masterpiece no less daunting, of course. The English National Opera face challenges that are superhuman - nay, positively Wagnerian: unending stamina from the singers; orchestra on nothing less than peak form; conductor in tyrannical, absolute command; director with ideas on what to do with a drama that has no drama at all. The Coliseum will be thronged with enthusiastic Wagnerians. But sceptics should go too. They may not get hooked - but they will at least be able to pride themselves on having sniffed, and escaped, the very distillation of Wagner's art.

! 'Tristan and Isolde': Coliseum, WC2 (0171 632 8300), continues Wed.