Peter Klaven, portrayed by Paul Rudd, is a dedicated real estate agent who has just become engaged to his thrilled girlfriend, Zooey Rice, played by Rashida Jones. Filled with excitement, Zooey quickly shares the wonderful news with her friends, Denise and Hailey. However, Peter finds himself in a predicament, realizing that he lacks close male friendships to celebrate this milestone in his life. His tendency to prioritize romantic relationships over friendships has left him with few guys to confide in.
In his quest to forge male friendships, Peter turns to his younger brother, Robbie, for guidance. He attempts to navigate various social situations, from attending a Major League Soccer game with an overly enthusiastic man named Lonnie to dining with a bisexually open Doug, and even having an awkward lunch with an elderly gentleman named Mel. His attempts escalate when he tries to join a poker night with Denise's husband, Barry, but after a mishap where Peter vomits on Barry, he is promptly booted from their gathering.
Just when Peter is on the verge of giving up, fate intervenes at an open house for a mansion previously owned by Lou Ferrigno. Here, he encounters Sydney Fife, played by Jason Segel, who is more interested in socializing with divorced women than buying property. A friendship blossoms between the two, leading to Sydney inviting Peter into his personal space—his "man cave," which is filled with an eclectic mix of musical instruments and memorabilia.
The budding friendship is put to the test as Peter introduces Sydney to Zooey at their engagement party. The atmosphere grows awkward when Sydney unintentionally gives an uncomfortable toast. Things take a turn when, while watching TV with Zooey one night, Sydney calls and persuades Peter to attend a Rush concert with him. Although reluctant to leave Zooey behind, Peter agrees, but the outing results in Peter focusing more on Sydney, leaving Zooey feeling neglected.
As the story progresses, suspicions grow within Zooey regarding Sydney's intentions, especially after an incident where he clashes with Lou Ferrigno, jeopardizing Peter’s work concerning the mansion sale. Despite lending Sydney $8,000, Peter finds himself questioning his choices and facing an emotional rift with Zooey, leading her to take a temporary detour to spend time with her friends.
In a surprising twist, Peter learns that Sydney has used the loan to launch an outrageous billboard campaign for his real estate business, which unexpectedly yields significant success. This professional boost empowers Peter to confront a confrontational colleague, ultimately reclaiming control over the Ferrigno listing.
As the wedding day approaches, Sydney grapples with loneliness while Peter continues to navigate his complicated emotions. Although Peter does not extend an invitation to Sydney for the wedding, Zooey senses Peter's yearning for his friend and reaches out, inadvertently paving the way for an unexpected reunion just before the ceremony.
In a grand flourish, Sydney arrives on a moped, revealing to the couple that he is, in fact, a successful investor and presents the billboard campaign as their wedding gift. With the tension resolved, Peter embraces Sydney as his best man, leading to a heartfelt wedding ceremony.
During the reception, the joyful atmosphere continues as Peter and Sydney join the band in a lively performance of Rush's "Limelight," even pulling Zooey on stage, while intertwining stories of love, unexpected twists, and humorous moments unfold around them. As the night draws to a close, a familiar sense of chaos returns when Sydney attempts to toast the newlyweds, prompting a frantic reaction from Peter as the screen fades out, humorously reminding the audience of their journey filled with ups and downs.
