It's hard to think of a more deceptive novelty hit than "Crazy", or one that hid so much in plain sight. Exuberantly propulsive and unshakably catchy, its singer, you eventually realised, really did doubt his own sanity. Gnarls Barkley remained anonymous as the single topped the UK charts on downloads alone. But when the curtain was pulled back, Cee-Lo Green stepped forward, an outsized Southern soul singer with a penchant for dressing in nappies, but deep pain in his past.

Father dead when he was two, mother paralysed from the neck down when he was 16, a street-gang robber, pyromaniac and military boot camp graduate, somehow Green is still seen as a jolly fat man. Those early, often outrageously funny shows, publicising a hit that children loved, prolonged the deception. But in this low-key, stripped-back return to London, he doesn't even look like he's playing now.

The band is in bow ties, brown jackets and shades, Green in loudly checked plus-fours, like a psychedelic high school hop band, innocent but wasted. Brian Burton, aka Danger Mouse, the most important producer in Britain right now, sits silently at the keyboards, smacking his leg with a tambourine. His work on the new Martina Topley-Bird, Black Keys and Shortwave Set albums, and Damon Albarn's The Good, The Bad and the Queen, have all been loudly acclaimed. But for this upstate New York vinyl addict, who moved to Green's Georgia at 13, having the latter's authentic 21st-century Southern soul presence at the front of his band must be a dream dwarfing everything else. The early assumption that Gnarls Barkley was his musical vehicle, fronted by Green is, anyway, sunk tonight. Burton is providing the ammunition for the singer to wrestle the world.

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"Have any of you guys heard these songs before?," Green suspiciously asks, as they finally debut the second album The Odd Couple, three months late due to unspecified personal problems. Commercial momentum has been lost, but he has bigger worries. "I'm not doing so good," he confides at the start on "Charity Case", in a voice sounding huskily raw and roughly used. "What do I do with all that aggression?" he wonders during "Just a Thought", lost in himself and his old life. When he dedicates "Going On" to "all the survivors", and sings either to or from his dead parents that "I'll be waiting for you", even the densest fan must realise that Gnarls Barkley is no longer a place for fun and games.

"I'm giving it my all, man," Green says before "Neighbors", when he really does. He is sitting, organ and cymbals softly shivering behind him, as he sings, with a dangerous, false smile: "My neighbour, he likes my clothes. But he doesn't see me with my scars exposed..." It is a song against materialism, envy and street stupidity, delving into the for-once legitimate pain of success. He tears his shirt open as it ends, once a symbol of jokey sexiness, now an expression of release.

"The Boogie Monster" (himself, of course) and "Blind Mary" (blind because she can't see he's "prettier inside") continue the psychic scalpel-work.

Respectful of their good luck and the song, Gnarls give "Crazy" their all, its descending groove making the fans swing like it's 1966. But on "A Little Better", Green, saturnine shadows and light playing on his big, still head, reaches the cleansing climax of the soul process. "I want to thank you, for hurting me so bad," he sings to his dead parents, as the band press down hard on their instruments, and a guitar wails. If Green only offered such self-absorption, he'd just be another dull singer-songwriter. But his voice takes you through pain, fulfilling soul's remit.

"Crazy"'s pop phenomenon is already fading. Gnarls Barkley, a 21st-century hiding place for a world Otis Redding would know, walk weirdly on.