The summer's most beguiling mystery isn't what those curious Prometheus aliens are up to, it's not Bane's plans for Gotham City. No, the most intriguing question of the season is what happened in a town outside of San Antonio back in the mid-1990s, when a thirteen-year-old boy went missing, only to astonishingly reappear in Europe four years later. Or so people thought. In the new documentary The Imposter (opening today in limited release) we quickly learn that the teenage boy believed to be long-lost Nicholas Barclay was actually a 23-year-old Frenchman known for posing as orphans and runaways. But he somehow made it all the way from Spain to Texas without any serious questioning of his identity, despite his different eye color and obvious accent. How did he pull it off?

That's the central mystery of the film, one that teases at questions of willful delusion and buried family secrets. Because we know very early on that this strange man, whose interviews with director Bart Layton serve as narration for the film, is a fake, we're less concerned with the ruse than we are with the psychology of self-deception, the desperate need to believe a whopping untruth. In some sense the imposter is not the man who turns out to be Frédéric Bourdin, The Chameleon, it's instead a belief, an idea, a refusal to acknowledge a truth pitifully hiding behind an obvious lie. Layton interviews Nicholas' mother, sister, and brother-in-law, all of whom are oddly both blunt and elusive. They seem like matter-of-fact Texans, straightforward and to the point, but then the narrative shifts a little, peeks around a corner and asks a new question, and their credibility, their seemingly flat honesty, is cast into doubt. This aspect of the movie is its real surprise — we think we are simply being told a story about a grieving family duped by a sociopath, until another, potentially far bigger, question sneaks up on us. Maybe these people aren't deluding themselves out of desperate hope. Maybe they're accepting the fake for far darker, more self-serving reasons.

Layton employs a recent documentary technique — seen in something like Robinson Devor's harrowing, profoundly unsettling Zoo — of mixing stylized, high-gloss reenactments in with the standard real-person interviews. I'm not sure how much the story is served by these staged interludes — do we really need a third Nicholas muddying up the proceedings? — but they do at least pad out what would otherwise be a fairly non-visual film. They can't compete with the brief snippets of actual home video, though. Seeing the real Nicholas and the fake one filmed by the same shaky camcorder is more unnerving than anything else in the film. We in the audience find Bourdin to be such an obvious fraud that to watch this family interact with the real Nicholas one moment and the next with the fake boy, all covered in a hat and scarf and sunglasses, is profoundly chilling. Bourdin-as-Nicholas told the family and the suspicious FBI agent in charge of his baffling case that he was the victim of a cruel and abusive sex trafficking ring, and it proves a smart story. It's so horrific that his family doesn't ask for many particulars. They let him be quiet and skittish and taciturn because they're convinced he's been incredibly traumatized by his ordeal. They let the lie flourish because the imagined truth is so unbearable.

Ultimately there is so much remnant ambiguity and uncertainty in the story of The Imposter that the film feels only like an intriguing glimpse into a bigger, darker story. We don't get much insight into Bourdin's psychology, only learning that he was neglected as a child and thus spent much of his adolescence looking for care and comfort. And Nicholas' family remains mostly obfuscated. There's a shadowy older brother who becomes a central figure of intrigue toward the end of the film, but he died of a drug overdose some years ago and so we're quickly met with a narrative brick wall. Whereas something like Andrew Jarecki's brilliant, equally mysterious Capturing the Friedmans used ambiguity as a central thesis, The Imposter uses it as a crutch; it gives up when the going gets murky simply because it got murky. There is more earnest digging to be done here, but Layton seems more concerned with making things stylish and moody than he does with tilling any real earth.

The subject of The Imposter is fascinating for sure, but it deserves more study. Layton has made an ominous, eerie film — a stranger-than-fiction spookfest that provides plenty of shivers — but for all the heavy notes of darkness and mystery, it feels oddly slight. The Imposter could have been a thorough sift through the records and rot of a particularly cursed American home. But instead it's merely a peek in through the windows; it points at a passing shadow on the wall only to quickly, teasingly turn away.