A gig by instrumentalist Chicago troupe Tortoise is usually a splendid event for several reasons, but there's one in particular that they are not always given credit for. Over the last few years they have graduated from the backrooms of Camden pubs to attracting what amounts to the fathering of the tribes, the ranks of fans swollen by everyone from techno-diehards, jazz evangelists and junglists as well as your average left-field electronic-rock obsessive.

And the reason, inevitably enough, is that Tortoise utilise all these elements with a coherent passion that many another band has tried to follow, mostly with little of the same magic. Their third album, TNT, at 65 minutes long, bears witness to their vast confidence. It's a veritable shifting tide of avant-garde melodies, light drum 'n' bass and picturesque electronica fused with jazz. As for those of us listening, a stamina for an ever-changing sonic landscape and an appreciation of technical proficiency would be a bonus, but this is far from a cold, prog-rock venture.

As it combines so many dimensions, TNT is a masterclass effort to be sure, but Tortoise's sense of adventure makes it fresh and the process is well conceived throughout - sometimes a bit of chin-stroking concentration on a band's part is a good thing.

Of course, some things have not gone Tortoise's way: some reviews of TNT from previously ardent journo fans have been negative, and their last London outing was sadly the final show for guitarist David Pajo, who has departed to work on his own band Arel M and Royal Trux. Part of his reason for leaving was a need for a freer, less perfectionist environment. And the crux is, Tortoise are as much a challenge to each other as to us listening. It will be curious to see whether they attempt the heavyweight continuous flow of music which joins three tracks on TNT for a 15-minute-plus opus. During this sonic wanderlust, the band travels through many a musical subculture, laying on a style that's a little cinematic, but displaying the technique of a fevered DJ in a club. After this brain food, you feel like awarding yourself a PhD.

Support comes from isotope 217, who also feature members of Tortoise, so it's a pretty heavy night ahead. A gig for those with bigger appetites for adventure than most.

Union Chapel,Upper Street, N1 (0171-226 1686) 3 Apr