OutKast is that rare thing in the arts: an experimental act that outsells most of its mainstream contemporaries. With their flamboyant costumes, avant-garde musical explorations and falsetto vocals, the Atlanta duo make unlikely poster boys for the macho hip-hop culture of the American South - yet that's the position granted them by 20 million record sales. Big Boi and André 3000, as they like to be known, certainly do not shy away from braggadocio. Unlike with other rap acts, however, their self-confidence fires up the music rather than the lyrics. They're also pretty hot on infuriatingly catchy pop hooks, as evidenced by their 2003 smash hit "Hey Ya!".

OutKast's new album, Idlewild, does not contain any such blatant pop potency. But it is a most beguiling affair: a 25-track journey through the musical history of 20th-century black America, driven by a 21st-century pulse. The album takes its inspiration from the pair's film of the same name, which is set in a speakeasy in Prohibition-era America. Thus it rolls through jazz, blues and big-band swing but also rap and funk.

The album opens with a skit from a spoof "Larry Olivier", recommending that acting be left to the actors, a maxim that OutKast gleefully proceeds to disprove. André 3000 goes on to inform us in his stylised twangy singing voice that "Life Is Like a Musical", while Big Boi sets his rapping against the bombastic brass of a marching band on "Morris Brown" - to great effect. On "Hollywood Divorce" they even persuade everyone's favourite misogynist rapper, Snoop Dogg, to discuss the value of marriage. The ghost of Larry will have to eat his words - the OutKast boys have created a hip-hop theatre all of their own.