The movie begins with Peter Klaven (Paul Rudd) pulling into an abandoned property with his girlfriend Zooey (Rashida Jones). He tells her about his plans for this place and says that if he lands his next client, a huge movie star, he will be able to afford all that as well as their reception. She looks at him, puzzled, until he gets down on one knee and proposes. Zooey happily accepts, and spends their entire drive home calling her girl friends on speakerphone. Since they don’t know that Peter can hear them, they begin to spill all the details about the relationship that Zooey has told them over the months. Peter is embarrassed, but just bears with it.
When they arrive at their home, Zooey asks Peter if he wants to give anyone a call. He tells her his parents are probably sleeping, so he can just let them know tomorrow. When she clarifies, saying that she meant any guy friends, he tells her he doesn’t really have any. This is further explained at dinner with Peter’s parents the next night. We learn that Peter has always had a girlfriend, and he always put them first. As a result, all of his guy friends ended up falling out. His father begins to joke about it, saying that even Peter’s gay younger brother Robbie has never had trouble making friends. His father takes it a step further, saying that Robbie is one of his two best friends. The other being a childhood buddy he still calls all the time.
The next day at work, we see that Peter gets along really well with all the women of the office, but when he goes to talk to Tevin, one of his co-workers, he’s really grossed out and annoyed. Tevin keeps telling Peter that he doesn’t know how to market himself. He’s too boring, and he’ll never sell houses with traditional advertising. Tevin talks boastfully about putting his face on buses and urinal cakes, effectively weirding Peter out. With his fencing partners, he finds that they don’t really consider him a “friend” because he’s never made an effort to hang out with any of them outside of the gym. Still, Peter is not truly bothered by his lack of male friends until he gets home early and overhears Zooey’s friends talking about how strange he is. They’re concerned that the wedding party will be unbalanced and they all say that guys without friends are too clingy, and that it’s weird that Peter’s best friend is his mom.
Knowing that he must make a change, Peter sets out on a series of “man dates” to find his Best Man. He learns the term from Robbie, who manages to make a married, straight man flirt with him in less than 5 minutes. The rules of the “man date” are a casual lunch or drinks after work, but NEVER dinner. Peter learns the reason for this rule the hard way as he goes out for dinner with a very amiable man his mother introduced to him, only to be surprised with an intense, all-out kiss at the end of the night. Robbie’s “date” for Peter is one of his gym clients, a soccer fan who has a very high-pitched voice. The date ends in disaster when the man tries to get the crowd to cheer but ends up getting them boo-ed by everyone else.
Zooey sets Peter up with her friend’s husband, Barry, and his poker friends. Barry is VERY reluctant to let Peter join them, and acts like a total jerk the entire time. He jokes about Peter not knowing how to play poker, then just when he’s thought he’s won, we find out that Peter’s hand beat his. They have a drinking contest and Peter wins, but since he’s not used to drinking so much, he throws up all over Barry and gets kicked out. Finally, Peter goes to meet a man from the internet, only to find that the man used an old photo and is now well into his eighties.
Disappointed by all the date disasters, Peter’s just about given up when he hosts an open house for his client and meets Sidney Fife (Jason Segal), a very blunt and friendly guy. Peter is, of course, awkward at first, but the two begin chatting freely when Peter points out that Sidney is the only person who’s eaten the sandwiches and Sidney compliments Peter on his panini choice. At the end of the conversation, Sidney admits that he’s only at the open house to check out women, and that he has no intention of purchasing the house. They exchange business cards anyway before going their separate ways.
Weeks later, Peter is going through a bit of a slump. The client’s house isn’t selling, and Tevin wants in on the property, but doing so would mean that Peter won’t get enough money to buy the land he wanted. He holds off, and after awkwardly practicing for a few minutes, gets the nerve to call Sidney up. He ends up leaving a very convoluted message, but Sidney calls him back anyway. They go to Venice Beach and Sidney tells Peter that he never picks up after his dog and shows him how to really yell to let go of stress. They have a great time together, and it isn’t long before the two are hanging out all the time. They put on jam sessions at Sidney’s and even go hiking with some of Sidney’s old friends.
Meanwhile, Zooey is worried about Peter spending so much time with Sidney because Peter has started ditching their weekly HBO nights to hang out with him. Peter invites Sidney to their engagement dinner. They try to set Sidney up with one of Zooey’s friends, but she is giggly and sporadic the entire time, and Sidney is completely uninterested. At the end of the dinner, Sidney makes a toast praising Peter and suggesting that Zooey “pleasure” Peter more often. The drive home is awkward and silent, with Peter finally coming out and admitting that he told Sidney about the lack of oral sex. They discuss it, and Zooey tells him that she’s not against it. In fact, she enjoys it, but her last boyfriend didn’t, and that’s why she avoids it. Peter tells Zooey not to be angry at Sidney because if it weren’t for his toast, this topic never would have come up. She agrees, and they make up. Zooey thinks that her friend got along very well with Sidney and that they should go out on a group date, so Peter agrees, even though he knows that Sidney hates going on these kind of dates.
Peter sets up a golf date for them, and although Sidney agrees to go, the day ends in disaster, with Peter and Zooey returning home angrily. Later, Sidney calls Peter up and tells them that Rush, their favorite band, is putting on a show tonight. Peter is about to decline, saying that it’s HBO night, but Sidney convinces him to go. Peter brings Zooey, but ignores her the entire night, choosing instead to air-rock with Sidney. Zooey is visibly confused and upset with her fiancé’s behavior.
Peter takes Sidney with him to buy the tux for the wedding, and although Sidney wants Peter to wear a blue suit, Peter doesn’t know if he can pull it off. Sidney asks Peter how their relationship is, and Peter admits that it’s been rocky lately. Sidney then asks Peter why he’s marrying Zooey, and Peter can’t come up with an answer. Sidney also asks Peter for a loan of $8000 for an “investment project”. Peter is reluctant, but agrees. As they are about to leave the store, the man who kissed Peter on one of the man dates earlier comes up to them and tells Peter off. After explaining everything to Sidney, Peter asks him to be his Best Man. Sidney is honored and gives Peter a huge hug just as the man walks by again. Later, Sidney and Peter are walking along when they see Peter’s client talking with Tevin. An enraged Sidney runs up to them and begins taunting the client, who’s buff enough to wrestle Sidney and puts him in a sleeper hold. Peter apologizes profusely before leaving, but is unsure of whether or not he still has the client.
That night, Zooey and Peter have a fight when Zooey finds out about the loan to Sidney. Peter asks Zooey why they are getting married, and she tells him that until he figures it out, she’s going to go stay with her best friend. Unsure of what to do, Peter goes to Sidney’s to break off the friendship. On the way, however, he spots some outrageous billboards. All of them have Peter in ridiculous poses, mostly spoofing movies and whatnot. Peter realizes that this was Sidney’s doing, and becomes increasingly embarrassed as he drives. Once he arrives at Sidney’s house, he tells Sidney that they were really using each other for ulterior motives - Peter to find a best man in time for the wedding, and Sidney because all his high school friends were moving on, and Sidney was afraid of not having anyone to hang out with anymore. Peter is outraged about the billboards and says that Sidney basically ruined his life. Peter then tells Sidney that he knows why he wants to marry Zooey. It’s because one of the best nights of his life was spent with Zooey. They ate a summer salad, had some wine, and watched “Chocolat” together. Sidney doesn’t want to accept this and keeps arguing with Peter, but finally they go their separate ways.
Next, Peter goes to apologize to Zooey. He admits that he was wrong and tells her that he still wants to marry her. He also tells her that he’s broken up the friendship with Sidney, so she won’t have to worry about it anymore. When he returns to work, however, he realizes that the billboards were a hit! He has tons of messages, all from people responding positively to the unique advertisements.
At the wedding, Zooey’s bridesmaids look outside to see who Peter ended up choosing for his groomsmen. We see that he picked his brother, his father, his client, and all the men he went on man dates with earlier in the movie. Everyone agrees that they’ve never seen such a random group of men, and they share a laugh, happy that Peter managed to get some friends. When everyone heads inside, however, Zooey looks outside and she catches a desolate Peter staring out at the ocean and air-rocking with himself. Cut to Sidney on his moped. He sees a call from the hotel where Peter’s having the wedding, and he says that he’s surprised to be getting this call. Sidney is being re-invited to the wedding! He says he might not be able to make it, but when the camera pans out, we see that Sidney is already on the way, fully decked out in a tux. Meanwhile, everyone is standing at the alter, ready for the ceremony to begin, but since Sidney isn’t there yet, Peter asks his brother to hold the ring, a gesture Robbie appreciates deeply. They share a hug, and their dad agrees that both of his sons are his best friends.
Just as the pastor begins the ceremony, Sidney arrives on his moped. Peter is surprised, and we learn that it was Zooey who called Sidney. She wanted him to be there for Peter because it was so hard for Peter to make friends and after all, Sidney was a good guy. Sidney goes up to the front and gives Peter a check for the billboards, repaying him in full. Peter admits that the billboards really worked, and Sidney says it’ll be their wedding gift. He takes his place in line, and the couple is happily wedded. Peter and Sidney jam out on the stage as the credits roll.
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