It's hard to reconcile the torture of recording and writing with the colour and unexpected humour of their worldview. Andrew Harrison spends three months in Radiohead-world to discover the roots of their singular mission...

January 16, 2008: it is bitingly cold in the streets around London E1's Brick Lane, but the effect of a face-cracking wind are softened by the unfamiliar warmth of shuffling, closely packed bodies. It is Radiohead Secret Gig Day. This morning, the band announced on its website that it would perform at the new Rough Trade East shop that very evening. Bogus sicknotes flew through the electronic ether; people began to queue before noon. Now there's at least a couple of thousand squeezed into the cobbled streets, stamping and huffing into their hands, hoping for admission to a show with a capacity of a couple of hundred tops.

It is amazingly exciting, the brittle air fair crackling with anticipation at witnessing up close and personal a band who are usually just stick figures in some distant festival horizon, and disbelief that this is really happening. After the unexpected online release of In Rainbows, Radiohead have relinquished the role of modern Pink Floyd and instead become some reincarnation of the KLF, suddenly conscious that event and surprise are what it's all about. It's already clear that the Rough Trade shop will be too tiny for tonight's show. When flight cases are seen moving through the crowd, text messages fly and people flock like birds towards wherever they think the magic door is going to open. When we're ushered into hastily arranged substitute venue 93 Feet East over the road, ten minutes ahead of everyone else, the frozen crowd actually boos us. But in the club's main room I run into Colin Greenwood, Radiohead's bass player, who will shortly play before a human audience for the first time in two years. He's regarding the empty stage with the wide-eyed thousand yard stare of a condemned man.

The show, though, turns out to be one of the most astonishing things I've ever seen, simply because this sound that's so pored over but so little understood - this rock music that challenges itself without falling into the mirror-image cliches of 'challenging music" - is, for once, happening right in front of you. At last you can see how it's done. When Ed O'Brien stoops over a jury-rigged box of self-made effects and sound processors that's so close it's practically sitting at your feet, you can see him ministering to the buttons almost tenderly. Thom Yorke's electric scarecrow motions, usually framed by speaker stacks or conveyed by stadium Jumbotron, take on new immediacy when the droplets of sweat are arcing over your own head. He's not just putting it on. This is the way he is.

I used to think that Radiohead wrote and played their songs in conventional manner and then "Godriched" them afterwards - added the sci-fi stardust, the hallucinogenic textures and feedbacks and sonic wormholes with their producer Nigel Godrich. But now, on Videotape, O'Brien is plucking a guitar string to emit the sound of a twanging ruler, and I can finally see that whatever contortions Radiohead's music goes through in their famously tortured recording sessions, they really are doing with their very own hands. This is really happening.

Strangest of all, they're clearly enjoying it, big smiles of relief and pleasure flitting across the tiny stage and into the crush of people in front of it. This is alarming: Radiohead do not do fun. Except maybe they do. Halfway through the show - a full rendition of In Rainbows, in order, with an encore of older songs - Colin wanders to the lip of the stage grinning so hugely that the crowd actually starts laughing. They finish In Rainbows, encore with My Iron Lung and The Bends and when Radiohead go offstage it seems they don't want to leave.

The world is full of overheated, fake once-in-a-lifetime events. Tonight the crowd ebbs away into the frosty night of east London's Nathan Barley-land, wrapped in the happy daze of people who've just witnessed a real one, and know it.

Back in the days when I went to the pub on a Friday night instead of hurrying home, Colin Greenwood occasionally came too. He had gone to Cambridge with a friend of mine, and every six weeks or so Colin would join us for a beer, his appearances becoming less and less frequent with the rising fortunes of what we all still called "Colin's band". One night, back at another friend's flat in Stockwell after last orders, Colin sheepishly asked if he could play a cassette of songs that Radiohead had just finished. They'd been working on it for ages, he said, probably for too long, and though they thought it might be good they really couldn't tell any more. Would anyone mind if he played it? He put the tape on and wandered off to make a cup of tea. This is how I first got to hear the songs that made up OK Computer, a record that would soon be routinely and without irony acclaimed the best album ever made. The man who played bass guitar on it seemed a little bit embarrassed about the fact.

I remind Colin of this as we meet in the discreet lounge of the Old Parsonage Hotel in Oxford, one afternoon in early March, and he laughs, sheepish again. Chronic insecurity is part of the Radiohead public persona, even if they can't possibly have it as bad as they seem. In fact Colin is, and always has been, playful and funny, if a little off-centre. He orders us the "Very High Tea" - fancy sandwiches, fancier scones. "That's a statement teapot," he says, impressed, when the tea arrives. The pot is of such sophisticated modern design that when Colin pours, the tea goes everywhere.

Now married with two sons, Colin is in his 22nd year as a member of the band that formed at Abingdon School -  he and his family still live nearby -  and was until 1991 known as On A Friday. I ask him how he'd describe Radiohead to someone who had no concept of rock and roll. That's easy, he says: "We got together at school and wanted to make music, but we had no formal concept of rock and roll either. We were kids from Abingdon, we liked Lou Reed, the Velvet Underground, The Fall, Joy Division, REM - all the people that are still out there -  but we didn't have any formal training and we didn't really know what we were doing. So we'd be exactly the same as the person asking the question."

Despite his nerves, Colin found himself thoroughly enjoying the 93 Feet East show. Beforehand Radiohead had worried that they'd completely forgotten how to play to an audience that was so close they could read the expressions on their faces. Instead it turned into another small step in the re-energising of the band, after an unhappy winding-down into predictability signalled by their last album, 2003's lugubrious Hail To The Thief, and the sudden delight of In Rainbows, whose recording was the usual endless purgatory for Radiohead but which arrived as fresh and beautiful as a spring morning -  in the middle of November.

He was particularly pleased at how Thom Yorke, the emotional bellwether of the band, had opened up during the show. Afterwards, Colin watched YouTube clips and he could see the singer doing his 'bobbly head thing", a hangover from the days when Thom's father taught him how to box. "Thom's got really broad shoulders," says Colin. "When you see them go down, boxer-style, that means he's really enjoying it." Then he smiles. "You know, I really hate it when people say, 'I'd love to do a tour of tiny places, get back to our roots.' Big tours are luxurious! I love them!"

Read Part Two.