Nev gets a bad feeling about it all when he realises that Megan has been lying about songs she has written and recorded when he finds them on youtube, sung live by others.
Things are worse when he calls an estate agent to learn that the art gallery the little girl Abby was meant to be moving into is still on the market.
He decides to drive across the US with his friends and confront the family, sensing that he has been led along in some bizarre game. Of course, just to make it clear for people not paying attention, they have a recorded chat with a waitress who also tells a story about people online not being who they say they are.
They jump in the car and first head to Megan's barn - which is deserted. He finds the mail he has sent there directly still in the mailbox.
Next they end up at Megan's family home in that town with the strange name. They rig up Nev with hidden microphones, which pick up sound perfectly without any of the usual background noises or wind resistance, and they decide to take it easy on Angela.
Angela is home and surprised to see them. She has two disabled step-sons, busy at work painting, and a husband - and they don't resemble their profile pics all that much. Conveniently Megan is not home, but they can meet Abby who is at a beach house with a young friend - alone - which is weird.
They go to see the young artist, who passes on that she doesn't paint and hasn't seen Megan for a long time.
In a private chat with Angela while watching Abby ride a horse, she confesses in a slow way that she was the person talking to Nev on Facebook all along. Back at her house, we then hear that Megan is at a place for alcoholics, and it was Angela who pretended to be her daughter on the phone. Surprisingly, Nev doesn't seem all that shocked or close to vomiting everywhere when hearing this.
Vince, the husband, says that he was pleased Angela had a Facebook friend and someone who was interested in her paintings. He then tells a story about catfish - which is where the movie gets the title.
Angela soon lets on about all the other fake Facebook profiles she made to keep the story alive, while she sketches a patient Nev. The pictures of Megan are apparently those of a distant cousin.
The lads say their goodbyes and leave to go back to New York.
The ending titles - we find out that one of the disabled twins has since died, that Angela does not have breast cancer, that Megan is not in rehab and instead has just left home, that the girl in the Megan pictures was not a family friend but a model with a portfolio online, that Angela has decided to take her art hobby more seriously, and that she has deleted her other Facebook profiles. Nev showered for three weeks straight.