Aimee Mann has long been a storyteller. In a bold - not to say eccentric - move, film director Paul Thomas Anderson based his 1999 movie Magnolia around her songs. The resulting soundtrack proved to be a high point for the Virginia-born songwriter although, as she wryly points out tonight, she was pipped to the Oscar by Phil Collins "and his monkey love song".

Her new album The Forgotten Arm sees her at it again. It tells the story of a girl who travels around with the state fair and her lover, who returns from Vietnam as a heroin addict. Live, Mann decides against playing it all, instead giving us selected songs with helpful announcements that begin: "At this point in the narrative ..."

This does help clear up exactly what Mann is singing about, but also gives the gig a slightly plodding feel. The music doesn't help. Most of Mann's songs are pitched somewhere between an amble and a wake. Sugarcoated may have spacy keyboards and thudding percussion, but before long it has slipped into an easy midwestern chug. At times like these, Mann sounds like Sheryl Crow without the curls, while she and her band - all of whom stand stock still - have all the theatricality of a breeze block.

She fares far better when she strips things down. Wise Up is gorgeous, with Mann's slim, nimble voice at last given room to breathe, hovering touchingly over a sparse piano and guitar. Elsewhere there is much to enjoy, particularly from Mann's relaxed banter: "Never put your parents on the guest list," squashes one heckler flat.

But it's not till the encores that she really gets going. An unrehearsed Susan is sweet and flawed, 4th of July comes with a dangerous edge, and closer Deathly even sees her band pull some rock shapes as they work their way up to an epic guitar solo. If Mann thought less about narrative and more about audience thrills she might just have a cracking show on her hands.

· At Shepherd's Bush Empire, London (0870 771 2000), tonight and tomorrow.