John Beckwith, played by Owen Wilson, and his friend Jeremy Grey, portrayed by Vince Vaughn, work as divorce mediators in Washington D.C. Taking their leisure time seriously, these two charmers often crash wedding receptions to meet women, adhering to a set of guidelines passed down to them by a previous wedding invader, Chazz Reinhold. Their approach always comes with a trusty story to evade nosy guests, ensuring they are the life of every celebration. Their primary objectives? To indulge in the free food, enjoy the flowing drinks, and seduce unsuspecting bridesmaids for a night of passion.

After several successful escapades, Jeremy introduces John to an extravagant wedding for the daughter of U.S. Secretary of the Treasury, William Cleary, played by Christopher Walken. Inside, they find themselves interested in the Cleary sisters, Gloria and Claire, with Claire being portrayed by Rachel McAdams. Jeremy finds adventure in the setting, ultimately bringing Gloria to a nearby beach for an intimate encounter. However, following their meeting, Gloria becomes unnervingly infatuated, shockingly proclaiming she was a virgin until their encounter, leaving Jeremy anxious and desperate to escape the party.

Meanwhile, John entices Claire, the maid of honor, but faces interference from her volatile boyfriend, Sack Lodge, portrayed by Bradley Cooper. Sack’s intentions become suspicious as he shows a blatant disregard for Claire, treating her poorly while being drawn to her father’s influence. Later, John cleverly convinces Jeremy to accept an invitation for a weekend retreat at the Cleary family estate. Here, family games lead to a tumultuous sequence of events, including a rough football match where Sack injures Jeremy, while Gloria, bewilderingly intent on pursuing intimacy, offers comfort to Jeremy.

While having dinner, uncontrollable passion arises as Gloria attempts to seduce Jeremy under the dinner table, while John enacts a clever disruption by spiking Sack’s drink. Sack’s resulting illness provides John the chance to connect further with Claire on a romantic walk. However, John’s evening takes an unexpected turn when William’s wife, Kathleen, makes an advance that leaves John disoriented. The night spirals further as Gloria’s intensity peaks when she seeks to tie Jeremy up in what she views as an exciting endeavor, only for it to border on assault, while Todd, Gloria’s brother, openly expresses his interest in Jeremy, creating comedic chaos within the Cleary household.

The next day, despite Jeremy’s reluctance, John persuades him to stick around, realizing that his own feelings for Claire deepen. An afternoon sailing trip burgeons with connection between John and Claire, culminating in a revealing conversation regarding her uncertainties about Sack. Their chemistry escalates into a passionate kiss at a secluded beach. Meanwhile, Jeremy learns the truth about Gloria’s past as a non-virgin, causing him to rethink his feelings.

As events unfold, Sack unexpectedly announces his engagement to Claire, leaving her in a state of confusion. Outside, just as John is about to profess his feelings to Claire, Jeremy’s dramatic chase interrupts the moment, forcing John and Jeremy to reveal their true identities as wedding crashers, leading to their nonchalant expulsion from the Cleary home. As they head out, Jeremy decides to keep Todd’s artwork as a gesture of kindness.

Returning to their everyday lives, Jeremy tries to maintain a secret relationship with Gloria while John finds himself heartbroken over Claire, compounding his despair by crashing weddings alone, leading to wild and reckless behavior. Jeremy’s relationship with Gloria progresses when he surprises her with a marriage proposal. As Jeremy attempts to reconnect with John, John’s encounters with a certain mentor, Chazz, compel him towards reconsideration of love.

During a funeral crashing adventure guided by Chazz’s peculiar advice, John’s viewpoint shifts when he witnesses genuine grief. This newfound realization pushes him back towards love as he races to interrupt Jeremy’s impending wedding. John’s dramatic appearance at the ceremony delights Jeremy, but quickly spirals out of control as John declares his love for Claire, inciting an emotionally charged confrontation with Sack. In a display of familial loyalty, William ultimately backs his daughter’s choice to refuse Sack’s demands.

The culmination sees Jeremy and Gloria uniting in matrimony, while John and Claire declare their love for one another, embarking on a shared journey as the four leave to contemplate crashing their next big event together.