Nelly and the St Lunatics , Wembley Arena , London; <br></br>N*E*R*D featuring Justin Timberlake, Brixton Academy, London

Not on your Nelly. Not when you've got the Neptunes

If you had a room of 15,000 people in the palm of your hand for one night, what would you make them do? Play the world's biggest game of Twister? Conduct an experiment in mass telekinesis? Raise a people's army and seize control of the state? I only ask, because when the Elastoplast-faced St Louis rapper Nelly finds himself in this position - having just brought the house down with a rendition of the wonderful "Work It" (sans Timberlake, of whom, more later) - the only thing he can think to do with all that power is to instruct us to throw our hands in the air and, furthermore, to wave them like we just don't care.

It's a moment which, with its soul-destroying predictability, encapsulates a disappointing evening. Now, I've got a lot of time for Nelly, which in hip-hop circles is becoming a capital crime. This backlash is largely due to "Dilemma", the soft, wet ballad which, like LL Cool J's "I Need Love" all those years ago, threatens to permanently derail his credibility. Well, I happen to like that soft, wet ballad, and I found it difficult to dislike the balladeer. It's the way he acts in the video, standing behind Kelly Rowland with his arms around her, grinning in disbelief: "Look what I've got! One of Destiny's Child! A no-good thug like me!" This, of course, is why St Lunatics - the crew he was with before he ever made it solo - are so important to Nelly. Like Eminem's D12, they give the impression that he isn't a loner, that he rolls deep. With their conveniently geographical name, they also show the world of hip-hop, which sets so much store by "representing", that he's still all about "the hood", he's still Nelly from the block.

Part of the problem of tonight's show, however, is that he gives his crew too much exposure. Romping around in front of a giant mirrored Derrty Ents sign and an arc-shaped screen (presumably intended to echo the Gateway Arch) in green ice hockey shirts to distinguish them from Nelly's blue - it's interesting, incidentally, that young black Americans are so keen to wear the attire of a sport in which they, by and large, do not participate - there are, frankly, too many of them, with too little personality to go around.

Sadly, the main man (whose trademark sticking plaster, originally intended as a symbol of support for his imprisoned brother, is gone from his cheek, after it became too much of a fashion item) doesn't want the spotlight, or perhaps feels he cannot do it justice. A scattering of Nelly's own tunes - "Andale Andale", new single "Iz U", and "Pimp Juice", the name of Nelly's energy drink which was boycotted by many African-American groups for being demeaning (and you can see their point) - are punctuated by bastardised covers of MOP's "Cold As Ice", Eve's "Let Me Blow Ya Mind", and an elongated interlude in which the dancing girls, in crappy crop tops and white trainers, shake their stuff one by one, not particularly well, to Sean Paul and Beyoncé hits.

"Hot in Herre" is, of course, a fantastic single, horny and funny at the same time. But it's ruined by the sound which, for such a major venue, is shockingly poor. Bassy and distorted, it's just a load of shouting and thudding, like a cheap car stereo turned up to 11. The mixing desk desperately needs the genius touch of the Neptunes (with whom most of Nelly's best work was done), but unfortunately Chad Hugo and Pharrell Williams are across the other side of London.

Even in normal circumstances, a gig by N*E*R*D, the soul-rock band alias of the Neptunes, the production team responsible for changing the course of turn-of- the-millennium pop, would be unmissable.

About three songs into their Brixton Academy show, it suddenly turns into a major musical event. Without any great pomp and circumstance, a white man in a yellow wool skullcap and a maroon collegiate T-shirt sidles up to the alarmingly skinny Pharrell (can't those millions of Britney dollars pay for a few pies?) and starts to sing. It's a few seconds before a word of mouth whoop, first low then rising to shrill hysteria, spreads around the theatre, when everyone realises that this man is Justin Timberlake.

Opinion is split on Timberlake - talentless boy band chancer, or OK-to-like purveyor of great pop - but the latter argument owes it all to the Neptunes. And if anyone knows this, JT must, which is doubtless why he's happily assented to help them out tonight.

Normally, a superstar cameo lasts for one or two songs, before they run off to their stretch limo and the crowd settles down again. But tonight, Timberlake stays onstage for the entire show, effectively as Pharrell's co-lead singer, refusing to hog the limelight, standing at the back when it's time for a human beatbox, or the fearsomely tight backing band Spymob, to do their stuff (although his false modesty almost backfires when a girl contestant in an audience participation section runs directly to the wings and starts dirty dancing with him).

The falsetto voice is one of the great paradoxes of soul music. This high, pure, innocent sound - often called castrato or, if you wish to be blunt, "eunuch" - is implicitly sexless, and yet it is a phenomenally effective vehicle for conveying feelings of carnal desire (which is why the back catalogues of Al Green, Prince and Curtis Mayfield continue to sell). Falsetto voices are the fallen angels of soul. To have two of modern pop's leading falsettos - Williams and Timberlake - in the same room at the same time feels like heaven itself.

Hearing N*E*R*D's entire In Search Of... (culminating with a pulverising "Lapdance"), plus assorted other Neptunes-related hits ("Frontin'", "Hot in Herre" for the second time in two nights), and Timberlake's own "Rock Your Body" and "Senorita" (offering a thoroughly enjoyable opportunity to yell along with the guys/ ladies call-and-response section) adds up to one of the best gigs I have ever witnessed.

If we must have vanity projects from studio-bound record producers, then let them be like N*E*R*D. And if we must have confused young pop stars - on the one hand, he helps fight the good fight by explicitly calling the CIA "terrorists"; on the other, he endorses globalisation by advertising McDonald's - then let them be like Justin.

s.price@independent.co.uk

Nelly: Sheffield Arena (0114 256 5656), tomorrow; NEC Arena, Birmingham (0121 780 4133), Tuesday