Jamie is a con-man who pre-arranged the events in his documentary and befriended Josh and Cornelia simply to meet Cornelia's father. Nobody but Josh cares about Jamie's duplicity and he goes on to have a successful career. Josh and Cornelia reconcile and decide to adopt a child.
Josh (Ben Stiller) is a married man in his 40s. He was an emerging documentary filmmaker but has been stuck making the same documentary (the person who is the subject of the documentary is played by Peter Yarrow of the band "Peter Paul & Mary") for over a decade. He is married to Cornelia (Naomi Watts), who is in her late 30s. Cornelia's father Leslie (Charles Grodin) is a highly successful documentary filmmaker and Josh's former mentor. Josh and Leslie have had a falling out. Josh and Leslie's best friends Marina and Fletcher (respectively played by Maria Dizzia and the Beastie Boys' Adam "Ad-Rock" Horovitz) have just had a child and have little time for Josh and Cornelia. Cornelia has had a couple of miscarriages and the couple have decided not to try for any more children.
One day, Josh meets a young (early-20s) married couple named Jamie and Darby (respectively played by Adam Driver and Amanda Seyfried). Jamie is an aspiring documentary filmmaker who idolizes Josh's films. Slowly, the two couples become friendly. Josh and Cornelia come out of their shells and try many new things. This culminates in them blowing off a weekend with Marina and Fletcher (and many other new parents) to attend an Ayahuasca ceremony where they drink a hallucinogen that also makes them puke. At the party, Josh decides to help Jamie with his next documentary. While high on the drug, Cornelia accidentally makes out with Jamie while thinking he is Josh (though she doesn't immediately stop making out with Jamie after realizing her mistake). Later, while at lunch with Jamie, Josh bumps into Leslie and introduces him to Jamie. Leslie offers to arrange a meeting for Josh with an investor so he can get the funds to finish his documentary and Jamie gets Josh to accept the meeting (though the meeting goes very poorly).
Jamie explains to Josh that he wants to make a documentary where he joins Facebook for the first time and goes to personally visit the first person from his past that he no longer talks to and record the journey. Josh, Darby and Cornelia tag along with Jamie on this endeavor. They meet a childhood friend of Jamie's named Kent. They find out Kent has attempted suicide and is now in a mental hospital. On camera, Jamie gives a stirring speech about how he lost his mother as a child and how Kent helped him get through it and resolves to help Kent now. Josh later does a Google search on Kent and finds out that Kent is a former marine who was injured in the war (which very likely led to Kent's current depression). By sheer happenstance, Jamie has stumbled onto a great story for his documentary.
Josh finally makes a cut of his film and, swallowing his pride, shows it to Leslie. After watching the film Leslie (correctly) tells Josh it is a bad film and that Josh has squandered his talent. Josh storms out. He later attends the premiere of Jamie's documentary. He is surprised to find Leslie there. After the restaurant meeting, Jamie kept in contact with Leslie and has since convinced Leslie to mentor him. Josh also learns that Cornelia made-out with Jamie at the ceremony and is now producing his film. Josh storms out in a fit of jealousy. He later fights with Cornelia and begins sleeping at Marina and Fletcher's house.
Eventually, Josh takes Leslie's advice to heart and begins re-editing his documentary . His editor (also working on Jamie's film) is closing some clips from Jamie's film and Josh notices something that contradicts Jamie's claims that he hadn't been in touch with Kent since their childhood. Josh tracks down Kent and realizes that Kent was actually Darby's childhood friend. He realizes that Jamie already knew of Kent's tragic backstory and simply manipulated the situation to make his "discovery" seem more compelling. During a confrontation with Darby (wherein she tries to make out with Josh, and he rejects her), he learns that the story Jamie told about his mom was actually about Darby's mom (as far as we know, Jamie's mom is still alive). Darby confirms Jamie is essentially a fraud who used Josh to get to Leslie. Cornelia separately comes to this same conclusion about Jamie after noticing discrepancies in his own statements to her. Darby subsequently leaves Jamie.
Josh confronts Jamie about his lies at an awards dinner for Leslie (featuring a cameo by Peter Bogdanovich). Jamie doesn't deny his manipulations, but simply believes that his distortions are not a big deal as Kent's story is still real. Josh exposes Jamie to Leslie, who ends up taking Jamie's side and noting that Jamie made a great film. Josh (who has always valued the process of film making more than the finished product) leaves the dinner. Cornelia goes after him and though she admits that Jamie made a good film and that his lies don't matter, the pair make up.
The film ends some time later with Marina and Fletcher dropping Josh and Cornelia off at the airport. It is revealed that they have decided to adopt a child from Haiti and are flying there to pick the child up. As Cornelia flips through a magazine, she comes across an interview with Jamie, showing that he has made it big in the film world. The film ends on an ambiguous note of whether, despite their maturation, Josh and Cornelia are ready to be parents.
Thanks Evan B!