Surprise is a rare thing in the movies in the Internet age. So far be it for me to spoil the one at the heart of “Catfish,” an indie documentary that’s being promoted as a sort of “Blair Witch” style thriller — maybe without the witch. Seriously, the trailers and ads for this Sundance darling would have you think there are shocks, jolts and moments where characters fear for their lives.

That’s a come-on, and if you go in expecting that, you’ll be bitterly disappointed. But what New York filmmakers Ariel Schulman and Henry Joost have cut together is a mildly suspenseful documentary, a game of Facebook Liar’s Poker that amounts to a cautionary tale for the  No Privacy generation.

Schulman’s younger brother, Yaniv, strikes up a friendship with a kid who is sending him colorful paintings based on his photographs. The painter’s name is Abby. She’s eight years old and she has talent. They exchange emails, and more emails. Yaniv — he goes by “Nev” — is just a young professional being nice to and encouraging a fellow artist, a child living in a remote corner of Michigan’s Upper Peninsula. Nev chats, online, with Abby’s mom. And then he Facebook “friends” Abby’s hot-teenage sister, Megan, and things get complicated.

Using everything from Google Earth to Mapquest and Youtube, the film’s graphics capture this budding romance — showing us just how much both Nev’s life and Megan’s can be detailed, illustrated and investigated from the comfort of your laptop.  It’s a chilling lesson in just how much is out there about you, most of it stuff you have revealed about yourself, especially if you’re in an online social network or two.

The filmmakers, with Nev’s often reluctant participation, capture his growing fascination and the doubts that creep into his thinking. And they film (in various grades of shaky video, from sharp to grainy) a how-to primer for anybody seeking to check out what some stranger is telling you about him or herself.  They also do a decent job of maintaining suspense, leading the viewer to expect some touch of Hollywood melodrama.

But nobody who has read the decades of stories about chat room trickery, or seen a certain Armistead Maupin true story film adaptation (starring the fellow who used to share the small screen with “Mindy”) will be dazzled by this film’s “surprise twist.”

Still, that total change in tone in the final act is gripping and effective even if we see it coming a mile (or a reel or two) off.

See for Yourself
“Catfish”

Cast: Yaniv Schulman

Directors: Henry Joost, Ariel Schulman

Running time: 1 hour 34 minutes

Industry rating: PG-13 for some sexual references.