Peter Klaven, played by Paul Rudd, is a dedicated real estate agent who finds himself on the brink of a new chapter in his life—he's just proposed to his delighted girlfriend, Zooey Rice, portrayed by Rashida Jones. As Zooey joyfully shares the news with her friends Denise and Hailey, Peter is struck by a disheartening realization: he lacks close male friendships. Throughout the years, he has inadvertently pushed away his male friends in favor of his romantic relationships, leaving him in urgent need of a best man for his upcoming nuptials.
Desperate to forge new connections, Peter consults his younger brother Robbie, played by Andy Samberg, who provides him with humorous yet unconventional advice. His efforts lead him to a range of quirky encounters, from a loud soccer fan named Lonnie to a dinner with the unexpectedly bisexual Doug and a lunch with Mel, an elderly gentleman. Even a poker night with the somewhat daunting husband of Denise, Barry, ends in disaster when Peter accidentally vomits on Barry after a drinking contest, which results in an unceremonious eviction from the event.
Just as Peter considers giving up on his quest for friendship, destiny intervenes at an open house event for one of his properties, owned by none other than Lou Ferrigno. It is here that he meets the charming Sydney Fife, played by Jason Segel, a man with a penchant for mingling with divorced women and a knack for witty banter. The chemistry between Peter and Sydney is palpable, and they quickly exchange contact information, setting the stage for a budding friendship.
As their camaraderie develops, Sydney invites Peter into his cozy "man cave," a sanctuary filled with various musical instruments and other personal items that signify their growing bond. At Peter and Zooey's engagement party, things take an awkward turn when Sydney’s clumsy toast creates an undeniable tension, especially since Hailey takes a liking to Sydney, who remains oblivious to her advances.
Subsequent nights reveal even more about their relationship, as Peter reluctantly agrees to attend a Rush concert with Sydney, which leads to an exciting bonding experience at the expense of Zooey feeling sidelined. During a shopping spree for tuxedos the next day, Sydney's requests raise eyebrows when he asks Peter for an $8,000 loan—money Peter ends up giving him followed by a cheerful invitation for Sydney to be his best man.
As the wedding draws near, Zooey’s suspicions about Sydney intensify, especially following an altercation involving Lou Ferrigno. Peter finds himself in a compromising position when Sydney utilizes the loan in an unconventional way—he creates billboard advertisements promoting Peter's real estate business. This unexpected stroke of genius leads to a surge of clients, emboldening Peter to stand up to his obnoxious colleague Tevin Downey in a satisfying confrontation.
However, the emotional tug-of-war continues as Peter grapples with his feelings regarding Sydney and his relationship with Zooey. With the wedding fast approaching, Peter decides not to invite Sydney, opting instead for a ragtag group of friends to fill the role of groomsmen, further tamping down his emotions.
Yet, when Zooey notices Peter's evident melancholy, she selflessly reaches out to Sydney, who is not far from their wedding venue. In a dramatic turn of events, Sydney arrives on a moped, revealing his successful investor status and returning the $8,000 as a wedding gift, transforming the day into a celebration of friendship and love. The couple’s heartfelt exchange solidifies their bond, and Sydney embraces his role as best man.
The film closes with a lively wedding reception, featuring an enthusiastic performance of Rush's "Limelight" with Peter and Sydney joining the band alongside Zooey on stage. As chaotic yet endearing moments unfold—like Denise’s unexpected news to Barry and a characteristically awkward toast from Sydney—viewers are left with a lingering sense of warmth and humor, exemplifying the complexities of friendship and love in this hilariously relatable journey.
