If your youngest son disappeared at the age of 13, you’d do anything to get him back. And if he re-appeared four years later on the other side of the world, mentally scarred after a horrific tale of sexual abuse, you’d pick him up no questions asked. But if his eye colour had changed, his skin was darker, he appeared older than 16 and spoke with an indelible foreign accent… could you pick out the truth from your desire to believe?

This is the story that unfolds in director Bart Layton’s incredible (and rarely has that word been more aptly used) documentary about the disappearance of 13-year-old Texan Patrick Barclay in 1994. A large family bereft by the loss of their blond-haired, blue-eyed boy, were only too willing to believe the news that someone claiming to be Patrick had appeared in Spain. His older sister – who had never left the US before, let alone able to find Spain on a map – promptly got on a plane to pick him up. Finding him much changed and very scared, she recognised some of his facial features and even the home-made tattoos he had on his hands and arms. He, she felt sure, was Patrick. And so she brought him home to a family who welcomed him back with open arms, a school that re-enrolled him and a local media who treated him as their darling.

Only it wasn’t Patrick. It was a 23-year-old European imposter called Frederic who – coming from a damaged background and backed into a corner by a lie that spiralled out of control – longed for nothing more than the loving family unit that the Barclays appeared to offer.

To tell any more of the story would be to ruin the film. For The Imposter is not just about the imposter himself, who narrates much of the film direct to camera, it is also about the family who embraced him, the small town characters that surround them, and – most importantly of all – about The Truth. We are constantly confronted with moral challenges: what would you do in this situation had you experienced what this person had experienced? What is real and what is fantasy? Who is telling the truth? Was the imposter’s fiction any less fake than the myths that keep families together? Just what is the glue that binds the fabric of America, the family unit?

The question marks that spill out of the film are endless. Not directly posed, but challenged of the audience merely by the story itself. By the end, you really don’t know which character you’re rooting for. As a thriller, it’s as gripping as The Bourne Identity (2002). It has a similar freshness too, using the language of its medium to trick you into thinking you’re in a safe place, that you’ve seen this before, only to be thrust into somewhere new and unexpected. Stylised cinematography and dramatic reconstructions take us beyond the reportage world of documentary and into a movie where fact and fiction are limited only by imagination. The town of Linares, Spain, feels like Sin City. It’s a very effective construct for, as we come to witness, Frederic’s imagination knows no bounds.

However the problem that any documentary maker has that action movie makers do not, is how to stay true to reality. Such are the twists and turns of The Imposter that is would be hard to over-egg the pudding; telling it straight would still have been gripping. Layton does an excellent job of heightening the drama in a way that enhances rather than detracts, enlisting the talents of cinematographer Erik Wilson fresh off the back of last year’s UK indie hit Submarine. But when the film veers off into a direction few could see coming (there were gasps in the press screening), so the mask of objectivity briefly slips. The finger of guilt is pointed toward a dramatic pronouncement that the facts to not back up. Some of the supporting cast too, from the local private investigator to the FBI liaison, could have been borrowed from an episode of The Simpsons and at times it does feel they are being played for laughs. Perhaps all this is a filmmaker’s prerogative. Perhaps not.

But there still won’t be many films this year that will leave you staggering out the cinema as dazed as The Imposter does. It is proof, if ever it were needed, that fact is stranger than fiction. It’s a powerful, entertaining and beguiling study of identity, love, loss, deceit, abuse, betrayal and hope. And for any documentary – or for any movie for that matter – that’s well worth the price of a cinema ticket.