As the sun sets over Los Angeles, a close-knit group of college friends gathers to celebrate the upcoming wedding of Elise and Michael. The atmosphere is charged with laughter and nostalgia as they partake in a series of events leading up to the wedding ceremony scheduled for the following day. Central to this reunion are Rich, the best man, and Samantha, the maid of honor; their relationship is a blend of enthusiasm and caution, especially since Samantha is harboring a secret — she is pregnant.
In the midst of it all, Sam Malone, affectionately nicknamed "Pockets" for his uncanny talent for pulling unexpected items from his pockets, returns from Thailand. He carries the emotional burden of an old love, Lana, and confronts her about their past, stirring up feelings of guilt and regret on both sides.
As the night progresses, the group splits into two factions: the guys head to Vegas on Simon's private jet while the girls remain at Elise and Michael's venue. Quentin, an energetic film agent, quickly forms a bond with Sarah, Elise's surprise-attending sister, who is actually only 17. Unaware of this crucial detail, Quentin's interactions with her sow the seeds for future complications.
Tensions escalate when Pockets engages in a heated argument with Simon, a self-centered movie star whose arrogance often belittles others. This confrontation leads to a shocking turn of events: Pockets finds himself arrested for soliciting an undercover cop, leaving the group's mood bleak and filled with concern for his safety.
Amid the tumult, Jennifer, Elise's former college girlfriend, connects with Lana over shared heartbreaks, leading to a night filled with unexpected intimacy between the two.
Meanwhile, Michael confides in Rich about his ambivalence towards marriage, viewing it not as a final step but as a chance for growth and commitment. He also reveals deep-seated issues surrounding his estranged father, Jonathan, who shows up uninvited, seeking redemption at the wedding. To Michael's shock, Elise admits that she invited Jonathan without knowing the full extent of their fraught relationship, forcing them to confront their joint secrets.
As dawn breaks, Michael visits his father's motel for a heartfelt reconciliation, an act that signals a pivotal moment for them both. He invites Jonathan to join the wedding, fostering a glimmer of hope for their future. Meanwhile, Lana decides to give Jennifer a chance, embarking on a new chapter that leaves Donovan, a colorful children's TV personality unexpectedly accompanying her, feeling sidelined.
In an unexpected twist, Quentin learns about Sarah's age but wisely opts to wait until she turns 18 to publicly embrace their relationship. For Rich, a momentous occasion arises when he proposes to Samantha, who at last reveals her pregnancy, marking a significant step in their journey together.
