Clips from Britney's show and reaction from fans who were there
Since Britney Spears's last UK concert in 2004, she has been married, had two children, got divorced, shaved off her hair, been admitted to hospital and taken her former manager and an ex-boyfriend to court.
Her recent TV appearances, including last year's X Factor performance, have seemed lacklustre and non-committal.
So the question on the opening night of her European tour was which Britney was going to turn up: the pop princess with the killer moves, or the disaster-struck diva in her slipped crown?
It was largely the former, "good" Britney. But it was a close call.
The theme for the night was the circus - a reference to the star's recent album, her fastest-seller in the UK to date.
Carnival rides had been erected outside the arena, while the daredevil acrobats of the Big Apple Circus provided the night's opening act.
When Britney appeared, descending from the ceiling, she was dressed in a red ringmaster's jacket, cracking a whip as she called her audience to attention.
The show features hits from across the star's decade-long career
"I'm a put-on-a-show kind of girl," she trilled in the opening song, and the girl didn't lie.
The concert was an unparalleled visual spectacle, full of magic tricks, gymnastics and costume changes (nine in all, including policewoman and burlesque dancer).
On stage were dozens of clowns, dancers, unicycles, acrobats, podiums, giant picture frames, period furniture and, at the centre of it all, the mistress of ceremonies herself.
But if Britney pulled out all the stops to divert people's eyes, it was only so they'd ignore their ears screaming, "she's miming!"
It's hard to be 100% sure, of course, but it wouldn't come as a complete surprise to learn that much of the 19-song set was lip-synced.
The band's contribution was hard to gauge too, tucked away as they were off the main stage, providing near-identical renditions of Britney's superlative robo-pop.
But does any of this really matter?
Not really. The audience weren't there to judge a music grade examination, they just wanted to witness the phenomenon.
Here was a girl they'd watched grow up, identified with, idolised and then prayed for as her life fell spectacularly apart.
At the O2, they were willing her on, hoping for a fairytale ending to that tragic story. And she did her best to oblige.
Britney was focused and engaged. The dancing sizzled and the 27-year-old's moves had regained their former snap and punch.
One early highlight was her two-finger salute to the tabloids, Piece Of Me ("I'm Miss bad media karma, another day another drama").
Britney started the song in a gilded cage, trapped like a performing circus animal. By the end, she'd broken free and taken control of her former masters. Subtle? No. But a great piece of theatre.
Piece of Me
Ooh Ooh Baby/Hot as Ice
Boys (The Co-Ed Remix)
If U Seek Amy
Me Against The Music
Get Naked (I Got A Plan)
Breathe on Me/Touch of My Hand
I'm A Slave 4 U
...Baby One More Time
When, during Touch Of My Hand, she spun into the air, blindfolded and suspended from the arm of a dancer in bondage gear, the flash of camera bulbs could have lit the Grand Canyon during a solar eclipse.
Elsewhere, Britney borrowed from Bollywood on a colourful Me Against The Music, and was sawn in half during Ooh Ooh Baby.
It wasn't until the halfway point, however, that the star remembered to address the audience.
It was profound, too. "What's up London?" she shouted, before launching into the night's sole ballad, Everytime, which she sang perched on the handle of a huge floating umbrella. Seriously.
If there was a fault to the set, it was the decision to focus on material from Britney's two most recent records at the expense of classics like Oops! I Did It Again.
As a result, the crowd never rose to its feet en masse until the show's closing one-two-three sucker punch of Toxic, ...Baby One More Time and Womanizer.
Meanwhile, the choreography - so tight and intricate at the beginning - began to drift into aimless posing and hip-swivelling.
Britney was a particular culprit here, strutting across the stage while her dancers put in the hard work around her.
So, in the end, she didn't quite give it her all.
But maybe it's a good thing that Britney has finally learned to hold something back for herself.
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