'Catfish' (PG-13)

“Don’t let anyone tell you what it is,” implore the ads for Catfish, the riveting documentary that was one of the sensations at this year’s Sundance Film Festival. The secrecy is not a matter of preserving a big surprise: There are no Crying Game switcharoos or Sixth Sense plot twists in store here. But knowing too much about Catfish beforehand ruins the experience, because the filmmakers discovered the movie as they were filming it, and a big part of its inexorable pull is in accompanying them on their journey and sharing their anxiety, unaware of what the final destination will be.

Things begin simply enough: Yaniv “Nev” Schulman, a 24-year-old photographer in New York City, receives a painting in the mail replicating a portrait of two dancers he shot for a newspaper. The artist claims to be Abby, an 8-year-old girl living in Michigan who, judging by the quality of her work, might be an undiscovered prodigy. She and Nev strike up a friendship via Facebook, and Nev’s brother Ariel, along with co-director Henry Joost, start to document their online relationship on film.

Nev also befriends Abby’s parents and especially her half-sister Megan, who has hundreds of pictures of herself on her Facebook account, is beautiful, single and openly flirtatious. She and Nev gradually begin to indulge in long telephone conversations and suggestive chains of text messages, without ever having met. Nev denies they’re technically dating, but he’s clearly infatuated with Megan, and eventually – inevitably – he starts pushing for a face-to-face meeting.

The rest of Catfish – which becomes curiously suspenseful and even disturbing in spots – should not be spoiled. The themes the movie raises, though, are fair game. The film is, among other things, an exploration of the exhibitionist streak in all of us that websites like Facebook have nurtured. At one point, Nev complains to his brother about constantly being filmed and claims to have been bullied into participating in the project. But although he is always free to pull the plug, Nev stays (and, near the end, seems more excited than the directors about what their cameras are capturing).

All of Catfish relies heavily on Facebook for its existence: This story would have never happened if the social-network site had not become so ingrained within our culture. And in its final 20 minutes, the film crosses over into territory that, from a certain perspective, could be interpreted as cruelly exploitative – a reality TV show starring people who never intended on having their lives filmed for public consumption (like The Truman Show, only for real).

But whatever your reaction to the moral and ethical decisions made by the filmmakers, you’ll find Catfish fascinating on a broader level as a snapshot of a rapidly changing era in which the more we Tweet and Facebook about ourselves, the more other people are compelled to do the same. Some critics have questioned the the film’s veracity, wondering why a handsome, intelligent young man living in New York City would embark on such an involved relationship with a woman he had never met (there are other issues in the film, too, that strain credibility).

But dismissing the movie as a work of fiction misses the larger point: Life in cyberspace has become so accessible and alluring, that people who just two years ago rarely used computers for recreation can now slip out of the real world and lose themselves in wall posts and status updates without thinking of the consequences. The title of Catfish isn’t explained until the end of the film, but it’s an ingenious capper for a cautionary tale about how the more technologically advanced we become, the more we need an occasional wake-up call to make us unplug, reboot and reconnect with reality. How many of your Facebook friends have you spent actual time with recently?

Directors: Henry Joost, Ariel Schulman.

Producers: Andrew Jarecki, Henry Joost, Ariel Schulman.

A Rogue Pictures release. Running time: 85 minutes. Vulgar language. Opens Friday Sept. 24 in Miami-Dade only: Sunset Place.