Jimmy Callahan, played by Kevin Hart, specializes in providing best man services for grooms lacking the necessary friends for their wedding day. When Doug Harris, portrayed by Josh Gad, a successful tax attorney, finds himself in urgent need of a best man for his upcoming nuptials to Gretchen Palmer, played by Kaley Cuoco-Sweeting, he becomes increasingly frantic. After a fainting spell, he is referred to Jimmy's company, The Best Man Inc., by his party planner, Edmundo.
Initially, Jimmy is less than interested in Doug's bizarre request for a "Golden Tux"—which requires seven groomsmen to match Gretchen's bridesmaids—a feat he claims to be "beyond" him. However, after hearing Doug's desperation and a lucrative offer of $50,000 plus expenses, he decides to take on the challenge. To mask their true identities, Doug adopts the alias Bic Mitchum.
As preparations for the wedding unfold, Jimmy assembles a ragtag group of former acquaintances to serve as groomsmen. This includes Fitzgibbons, a criminal on the run from federal authorities who jumps at the chance to meet bridesmaids, Lurch, a plumber eager to avoid his wife's nagging, and Reggie, an airline security officer looking forward to the promise of good catering. Together with his assistant Doris, Jimmy conducts interviews, leading them to select an eccentric crew of characters to fill out the rest of the groomsmen lineup, showcasing an array of unusual "party tricks."
When Doug excitedly introduces Bic to Gretchen and surprises her with honeymoon tickets to Tahiti, tensions rise. In a bid to fit in, Doug insists that Jimmy adopt the persona of a military priest from North Dakota. This leads to a disastrous family brunch where Jimmy has to concoct wild stories to maintain the ruse. An unfortunate accident involving Doug's grandma results in her needing a trip to the emergency room, adding to the chaos.
As the wedding weekend progresses, Doug's groomsmen, with their fabricated identities, partake in various staged activities to impress both Doug and Gretchen. They stage elaborate photo shoots that eventually lead to the groomsmen winning over Gretchen during a heartfelt slideshow presentation at the rehearsal dinner. Yet as the wedding draws near, secrets begin to unravel.
On the day of the wedding, after the family priest backs out, a desperate Doug concocts a plan for Jimmy to officiate the wedding in disguise. As the ceremony unfolds, chaos ensues with Gretchen, feeling overwhelmed, ultimately confessing to Jimmy her true reasons for marrying Doug. Overhearing this heartbreaking conversation leads Doug to reconsider the entire wedding. Just as Jimmy is about to deliver his best man speech, Doug interrupts to reveal the truth about their dishonest façade, setting off a series of comedic misadventures as they all scramble for the exit.
Ultimately, Doug pays Jimmy the agreed $50,000, and they develop an unexpected friendship that blossoms as they utilize Doug's first-class honeymoon tickets for an unforgettable trip to Tahiti, turning the wedding disaster into a lighthearted memory filled with laughter and camaraderie. In a humorous post-credits scene, Jimmy and Doug can be seen engaging in a hilarious game of tennis, showcasing that their adventures are far from over.
