Peter Klaven, a successful real estate agent, has a great life and a loving fiancée, Zooey. However, as they start planning their wedding, Zooey and her friends realize that Peter lacks a close male friend to serve as his Best Man. This revelation unsettles Peter, who then becomes aware of his lack of significant male friendships. Motivated to fill this gap before his big day, Peter embarks on a series of awkward and comical "man-dates" to find a best friend, ultimately leading him to feel more alienated.

During this quest, Peter meets Sydney Fife, an eccentric and free-spirited investor who crashes one of his open houses. Despite their seemingly opposite personalities, the two hit it off, starting an unconventional and tumultuous friendship. As Peter and Sydney spend more time together, their bond grows, causing Peter to neglect his relationships with Zooey and others close to him. This newfound friendship challenges Peter to understand the true meaning of friendship and to balance his life in a way he has never had to consider before.