Peter Klaven (PAUL RUDD) is a realtor who tries to be cool when the moment calls for it, but usually fails miserably and awkwardly. Even so, he's managed to attract Zooey Rice (RASHIDA JONES) as his live-in girlfriend, and he's hoping to sell the home of actor Lou Ferrigno (LOU FERRIGNO) in order to raise enough money to develop some land and thus support himself and Zooey to whom he's just proposed. The only problem is that he doesn't promote himself like his more gregarious coworker, Tevin (ROB HUEBEL), and hasn't had any success so far.
His unassuming ways haven't been lost on Zooey's sister, Hailey (SARAH BURNS), or best friend, Denise (JAIME PRESSLY), who can't help but point out that Peter doesn't have many male friends. Then again, Hailey isn't dating anyone, while Denise's husband, Barry (JON FAVREAU), is something of a jerk, especially toward Peter. In fact, Peter's best friend is probably his mom (JANE CURTIN), as his dad (J.K. SIMMONS) points out that their other son, gay physical trainer Robby (ANDY SAMBERG), is one of his two best friends.
Realizing he needs a best man for the wedding, Peter sets out to make some new male friends, but doesn't have much luck as he tries too hard and isn't particularly adept at being cool or hip. Even so, he ends up striking a friendship with investor Sydney Fife (JASON SEGEL) who habitually crashes real estate open houses for the free food and a quest to hook up with divorcees.
A straight-shooter, Sydney becomes best friends with Peter, but their growing friendship begins to put a strain on Peter's relationship with Zooey, particularly when it brings out another side of him that she's never seen before. From that point on, Peter must balance his relationships with both as his and Zooey's wedding date quickly approaches.