The new documentary Catfish, which follows a New York 20-something's strange relationship with online strangers, might seem to be about the cloak of anonymity provided by the Internet. It's not. It's about the new impossibility of being anonymous online.

Spoiler alert:I'll be discussing specific plot points of the movie and detailing a few riveting, if unsurprising, twists.

Catfish follows Nev Shulman, a 24-year-old New York filmmaker, as he becomes increasingly entangled with a faraway Michigan family he's never met. The action starts a few years ago, when Nev is emailed out of the blue by a woman named Angela. She asks him if it would be OK for her daughter, an 8-year-old art prodigy, to use a picture he took for a New York newspaper as the source material for a painting. Nev says yes. Soon after, Abby herself emails Nev to show him how well the painting turned out, and the two start an online correspondence, sort of a pen pal thing.

Nev eventually befriends the entire family (on Facebook), and starts a long-distance relationship with their 19-year-old daughter, an attractive, artistic girl named Megan. She records songs for him and attaches them to amorous emails, they speak on the phone, exchange sexy text messages and write on love notes on each others' Facebook walls. But every time they try to meet in person, something comes up.

As you've probably guessed, Catfish's big twist comes when Nev and his filmmaker buddies start realizing it's all a sham. They decide to pay a visit to Megan and her family in their small Michigan town to figure out what's going on. When they get there, they learn that almost every detail about Megan and her family was a lie. Abby, for example, isn't an art prodigy. She can't even paint.

Many critics have taken this as sort of a cautionary tale: So crazy! You never know who you're talking to on the Internet! That goddamn New Yorker cartoon with the dog is brought up. The Times' A.O. Scott writes

On the Internet, an ancient New Yorker cartoon caption observes, nobody knows you're a dog. But everyone assumes you're a sucker, susceptible to the pleas of hard-luck Nigerian royalty or eager to enhance your sexual prowess.

But what's most interesting about Catfish isn't that the Internet allowed a smart filmmaker to be bamboozled for months by a make-believe Michigan family. It's that the Internet allowed him to figure it out, track them down and make a movie about it.

Our collective Internet nightmare used to be that the cute 19-year-old girl we were chatting with was in reality a 34-year-old man. (Or a dog.) But in the new, densely socially-networked, Google-indexed Internet, it's pretty easy to figure out who we're really talking to. A nerd who might once have easily lied about his hot girlfriend in an AOL chat room now lies by posting a picture of his hot "girlfriend" on the social news site Reddit. (Pics or it didn't happen!) But then the real girl in the photo learns about it through Facebook and brutally embarrasses him.

The first sign in Catfish that something isn't right with Megan and her family comes when Nev discovers the songs Megan has been recording for him were actually lifted from YouTube. Five minutes of Googling brings Nev to the exact recording of "Tennessee Stud" Megan had moments ago passed off as her own, recorded at his request. The same enormous, instantly-accessible catalog of music on YouTube that let Megan deceive Nev also allowed him to easily cut through the ruse.

In fact the situation is almost completely reversed from the old New Yorker cartoon. What's weird now is how, on the Internet, Everyone knows you're a dog. What's scary is that just hours after a short surveillance video clip of a woman dumping a cat in a trash can was posted on the Internet, she was identified (thanks, in part, to Google Street View images of her block) and harassed by an Internet horde. Or that there exists encyclopedic Facebook records of our bad behavior that might someday surface and show the world who we 'really' are. (i.e. how much we really drink.)

The death of online anonymity was the problem Google CEO Eric Schmidt spoke to when he suggested that children might one day have to change their names to hide their youthful indiscretions. It's a different sort of Internet deception than that documented in Catfish, but it's just as creepy.

P.S. You should see Catfish! It's pretty flawed, but very interesting. And I think it's real.

[Photoshopped image of Nev and "Megan" together via Universal Pictures]