Other stars descend a staircase. Christina Aguilera rides on stage atop a moving one, a tiny vision in a white, silk suit, dwarfed by athletic dancers. A sassy, brass-heavy band strikes up Ain’t No Other Man, one of this summer’s standout singles.

Then she starts to sing and what should have been a storming start to a spectacular production falls flat on its face. Costume design by Roberto Cavalli, choreography by Madonna’s man Jamie King and sound mixed by a fish. The recently renovated Wembley may have done little to improve its notoriously tinny acoustics, but not even the venue can take the blame for this one. Ain’t No Other Man’s crisp, funky backing seeps soggily from the speakers, and the bob-haired Barbie whose voice could wipe the floor with every one of her size-zero peers wails as though she’s in pain.

And so it continues for the first 40 minutes. Back in the Day begins with a video montage of soul, jazz and blues legends and, boy, do you wish one of them was here now. Inspired by the faces flashing on screen — Billie, Aretha, Marvin — Aguilera’s current album, Back to Basics, has already proved a hard sell and tonight its highlights are few and far between.

One should be Understand, a power soul ballad for which Aguilera appears in her fourth outfit in four tracks — teensy shorts beneath a split-open skirt. Unfortunately, she sounds as if she’s on a hillside, battling a gale. But at least she sings. For much of the set she leaves that little detail to a trio of bluesy backing singers, preferring to hold high notes, body bent, head down, as though about to rush someone’s stomach.

Visually, the show is tremendous — slick, witty and furiously paced. Aguilera emerges from the floor draped over a piano, dancers do backflips from tables, and Under the Circus ushers on a Big Top set that includes acrobats on ceiling swings, a spinning stilt walker, female fire-eaters and the singer pinned to an enormous, knife-thrower’s target.

Finally, a few old favourites arrive and the sound perks up. Dirrty and Lady Marmalade get the crowd grooving for the first time and an ode to Aguilera’s mother, a ballad about domestic abuse, is a surprisingly moving moment. It was hard not to feel pity for the props, however. They deserved so much more.

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