Exit Through the Gift Shop, Banksy, 87 mins

Banksy speaks! (Sort of.) The elusive street artist caught on film! (Sort of.) Banksy's undoubted knack for exploiting the feverish interest his anonymity provokes has certainly created a lot of hype around this documentary.

The point is, it isn't really about him. It's more about the creation of another street artist, Mr Brainwash, and an exposé of the art market and "suckers" with too much money who want to be part of the latest thing.

It is beautifully layered, however, with a Postmodern self- referencing that sees a film within a film within a film. So what starts as a documentary about street art turns into a documentary about Banksy, and changes again into a documentary about the documentary before its blazing finale. It is also replete with the wit Banksy employs in his art.

So we begin with Thierry Guetta, a French-American who films every aspect of his life. He is drawn into the street-art scene through his cousin, Space Invader, whose thing is to paste up icons from the classic video game. This is before street art has become big business, when its ephemeral nature is being given permanence by photos on the web.

There is great footage of the early nocturnal shenanigans of artists who have since gone on to be collected – such as Shepard Fairey, best known for his Obama "Hope" poster.

The missing piece from Guetta's collection was Banksy. Eventually the two team up, become friends, and we are treated to glimpses of the artist (or rather the hands of the artist) at work – in his studio, infamously daubing the wall dividing Israel from the West Bank and, most amusingly, placing an inflatable Guantanamo Bay inmate in Disneyland's Rocky Mountain Railroad ride, which leads to Guetta being held for four hours by the Disney police.

The first completed documentary, however, is no good. So Banksy tells Guetta to go and be a street artist while he sorts the footage out.

Guetta takes him at more than his word. He takes on the moniker Mr Brainwash (MBW), sells his house and sets up a Damien Hirst-style operation, employing other artists to manifest his ideas. He creates hundreds of works of art, and publicises a massive exhibition in Los Angeles with a supporting quote from Banksy pasted on billboards.

He becomes front-page news, 4,000 people turn up to the exhibition, collectors ring up, he sells $1m-worth of art. We see him making up incredible sums on the spot. The trouble is, his art is no good. It is derivative of Banksy and Warhol, completely devoid of originality.

Exit Through the Gift Shop's first week is screened in a brick vault beneath Waterloo Station, quite literally underground (it reaches cinemas on 5 March). The tickets sold out in one minute and the entrance is through a graffitied tunnel. It's a great metaphor, but it also speaks to what is perhaps the film's only flaw – that it is born of its maverick world and therefore doesn't know its own power. None of the artists featured – apart from MBW – set out for mainstream success, which may be why their art speaks and why this chaotically charming film delivers.