Brian Wilson, Royal Festival Hall, London

This is the most ambitious exploration of the boundaries of pop music

"You guys in the audience have better cameras and recording gear than the people doing it officially," jokes Jeffrey Foskett, Brian Wilson's corpulent and genial bandleader. "And we expect to hear it on the internet tonight." Foskett knows whereof he speaks. The Beach Boys' unreleased 1967 masterpiece Smile is the most bootlegged, most sought-after, most mythologised album in the history of rock'n'roll.

Wilson once said that completing Smile would be "like raising the Titanic". His current keyboardist, Darian Sahanaja, calls it "the holy grail of rock'n'roll". By using old 1960s tapes and 21st-century computers, Wilson, Sahanaja and Van Dyke Parks, the lyricist who collaborated on the original project, have succeeded in that task.

Friday 20 February 2004 was the night the world finally got to hear some kind of definitive Smile. The show is topped and tailed by a couple of hits sets, and finds Brian in fine form, sitting behind his autocue/keyboard, clicking his fingers and frowning with concentration. His voice, never the sweetest, is as poignantly limited as it ever was.

But when Wilson orders the house lights up, and introduces "my friend, Van Dyke", and a small silver-haired man receives a standing ovation in the stalls, there's no question about the main event.

From the Hawaiian chants of "Prayer" onwards, Smile has a potent effect on the listener. Inspired by the young Wilson's growing fascinations with astrology, numerology and the occult, and bearing more in common with artistic movements than with the prevailing currents of pop, it remains, 37 years later, the most ambitious exploration of the boundaries of pop music. You feel as though your brain is about to explode with wonder at the possibilities of music. It's a feeling akin to a religious epiphany.

The three highlights are, inevitably, the full nine minutes of "Heroes and Villains", the beautiful "Surf's Up" and a staggeringly powerful "Good Vibrations" (with alternate lyrics). But Smile was never a song-based suite in the traditional sense. Skittish and episodic, the experience is like watching an orchestra performing a film score. There are sections lifted from Sinatra's "I Wanna Be Around", "You Are My Sunshine", "Barnyard" and "Vega-Tables" ("If you brought a big brown bag of 'em home/I'd jump up and down and hope you'd toss me a carrot"), both of which corroborate the theory that Smile was intended partly as a comedy.

As the opus ends, Van Dyke Parks smiles a smiley smile, points at Brian Wilson's heart, then at his own. They slap hands, and they're gone.