Catfish, Ariel Schulman, Henry Joost, 89 mins (12A)

This documentary – charting the director's brother's online romance with a woman who seems too good to be true – leaves you wondering who's the sucker, him or you?

Assuming Catfish is what it claims to be, American film-makers Ariel Schulman and Henry Joost have made a provocative statement about the question of truth on the internet.

The film, which appears to be a documentary, begins when Schulman's brother Nev, a New York-based photographer, receives a surprise package – a painting copied from one of his photos published in a magazine. The artist is apparently an eight-year-old girl named Abby.

More paintings follow, plus emails from Abby, full of charming details about her family in Ishpeming, Michigan. Nev corresponds with Abby and ends up making friends online with her mother Angela and older sister Megan, a beautiful young woman of prodigious talent – artist, dancer, musician, songwriter. The young photographer starts falling in (apparently requited) love with Megan, while his brother and Joost, who have decided to make a documentary about the whole affair, are just as enraptured by the remarkable Michigan clan, not least Angela ("That's the mom?" they marvel, admiring her on-screen likeness, "Wow! She's hot!").

By this point, most viewers will be thinking, "Suckerrrrrs!" – although the film-makers may be out to make suckers of us. I won't reveal too much, but Angela and family are indeed too good to be true – although it's bizarre that people as media-savvy as Nev and friends are late to catch on. A number of telling details – which they would have spotted if only (d'oh!) they'd thought to do a little routine Googling – prove phony, so the Schulmans and Joost travel to Michigan in search of the truth.

At this point, the film takes on a spurious thriller tone, playing on hoary myths about the weirdness of middle America: you expect Megan's homestead to be nothing less than the Bates Motel with broadband.

There are several reasons to be sceptical about Catfish. I can't buy the idea that Schulman and Joost are making a documentary about Nev's online passion, or that someone like him would be gauchely susceptible to e-flirtation in the first place. And I'm wary of the insistence with which the film-makers show their hand, forever reminding us that they're making a film: hearing one of Megan's tunes, someone chirps off-screen, "That would be a great song for a scene in this movie!" Equally dubious is the constant invocation of authenticity, e.g. the information that Abby puts her hair and spittle into her paintings for purposes of DNA verification. The film's own DNA looks more suspect at every step.

Maybe I'm being too sceptical – although that's seemingly just what the film-makers want. But I worry that we've reached a point at which we so routinely question the authenticity of documentaries that the whole issue of credibility and artifice has become stretched beyond the point of worthwhile interest. We've seen countless mock-docs that are fictions in disguise (The Blair Witch Project and its imitations); ostensible docs that turn out not to be docs at all (the Joaquin Phoenix prank I'm Still Here); others whose verity is yet to be proved or disproved (Banksy's Exit Through the Gift Shop). There's nothing new in the proposition that a documentary might prove to be a pack of lies – it was magnificently demonstrated by Orson Welles in his 1973 provocation F for Fake. But the whole topic may now have reached saturation point: you almost find yourself wishing, God forbid, that documentaries were obliged to come with an accompanying certificate of provenance.

Joost and Schulman want to make us think about gullibility, the unreliability of internet identity, and the kinds of fantasy and mutual delusion that online friendship entails. But Catfish muddies the water by not playing it straighter. Razzed up with fancy online-style graphics, the film comes across uncomfortably as a smug, slick novelty product. Its final section, which focuses on Angela, looks as if it's bona fide, the mystery replaced by a genuinely curious portrait of a highly unusual personality – a sad, even somewhat heroic woman who may finally be more real than the sniggering hipsters behind the camera. Everything that precedes, I'm convinced, is cooked in order to present us with an astounding yarn. But I couldn't say for sure – and when you find yourself asking, "What's for real?" too many times, the whole shebang evaporates into a bad case of credibility decay. Catfish could have been so much more intriguing and substantial, if not for the reek of red herring.

Next Week:

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