Peter Klaven (Paul Rudd) is a successful real estate agent who, upon getting engaged to the woman of his dreams, Zooey, (Rashida Jones), discovers to his dismay and chagrin that he has no male friend close enough to serve as his Best Man. Peter immediately sets out to rectify the situation, embarking on a series of bizarre and awkward 'man-dates' before meeting Sydney Fife (Jason Segel), a charming, opinionated man with whom he instantly bonds. But the closer the two men get, the more Peter's relationship with Zooey suffers, ultimately forcing him to choose between his fiancée and his new found bro.