Ross Noble, Garrick Theatre, London

At one point during his bravura two-and-a-half-hour show, Unrealtime, the stand-up Ross Noble compares his act to the interminable, logic-defying US TV series 24: "My name's Ross Noble, and this is the longest gig of my life." He goes on to apologise for its not being "the most linear show you'll ever see".

Indeed. But who needs to be linear when you're so effortlessly funny? Never adhering to a set routine, the black-clad Geordie comic is a master of taking tangents off tangents. He lives by the stage credo of: "I've started, so I won't finish." Continually trying - and failing - to return to stories he began more than an hour ago, his mind seems to wander as aimlessly as the clouds that are projected on to the screen at the back of the Garrick Theatre's stage.

The constant diversions matter little. With this comedian, the pleasure derives from the journey rather than the destination. At the first night this week, his comedy muscles toned by a sell-out run in Edinburgh, Noble is in tip-top shape. Much of his best material comes off the cuff. He is especially inspired when he discovers that a couple in the front row - not the best place to sit at a Noble gig - work for the Refugee Council. That information prompts him to start musing about why asylum-seekers always seem to wear jumpers with leather patches when they're interviewed on TV. Then, from nowhere, he recalls the graffiti that he saw scrawled in the grime on the side of a lorry in Dover: "Please overtake quietly - refugees asleep."

Later, Noble goes off on an equally original mental jaunt when he imagines that Tony Blair, having asked Barry White to sex up a dossier about weapons of mass destruction, is shocked to come in one morning to discover the gargantuan soul singer lying next to the dossier and smoking a cigarette. Inevitably, with this scattergun style, not every riff is on target, but for all that he has a pretty high strike rate. Even when an anecdote appears to be teetering on the brink of unfunniness, Noble usually manages to pull it back at the last moment with a killer line from out of the blue.

Sometimes he pushes his trademark mischief-making to the very edge of tastefulness - routines about David Blunkett and Stephen Hawking will have left some spectators squirming - but more often his cheekiness absolutely hits the spot.

Noble still has a low profile on television and has booked a month in the West End largely on the back of his burgeoning reputation as a live act. But the confidence looks justified. He may be only 27, but I predict a bright future for the comic who puts the "imp" into "improv".

To 27 Sep (020-7494 5085)