As the story unfolds, we hear the poignant voice of The Oracle (Penny Ryder), who reflects on the delicate nature of love and how a single moment can spiral into chaos. The narrative transports us to the enchanting landscapes of Italy, where Jack (Sam Claflin) and his journalist friend Dina (Olivia Munn) are on the brink of a life-altering opportunity. Just as they seem poised for romance, Jack's old buddy Greg (Alexander Forsyth) crashes the moment, leading Jack to abandon his plans and hang out with his friends.
Fast forward to the beautiful Italian countryside, where Hayley (Eleanor Tomlinson) is ready to marry Roberto (Tiziano Caputo). A colorful ensemble gathers to celebrate the occasion: Rebecca (Aisling Bea) and Bryan (Joel Fry) serve as best man and maid of honor; Amanda (Freida Pinto), Jack's ex, grapples with her insecurities; and Chaz (Allan Mustafa) supports her as a needy partner, while Sidney (Tim Key) adds a whimsical touch, sporting a kilt. Amidst the joyous exchange of vows, tension brews under the surface.
At the reception, Jack is taken aback when Dina makes a surprise entrance, fresh from a breakup, and invited by Hayley. An awkward conversation ensues as Sidney clumsily tries to engage Dina, underscoring the discomfort in the air. Meanwhile, Bryan is determined to impress Vitelli (Paolo Mazzarelli), hoping for a chance at stardom. Complications arise when Rebecca’s secret feelings for Bryan emerge, further stirring the pot. The drama escalates when Marc (Jack Farthing), an unwanted visitor from Hayley’s past, arrives under the influence, set on disrupting the wedding to rekindle his connection with Hayley.
Caught in the chaos, Hayley turns to Jack for help against Marc's antics. A cunning plan is conjured—Jack is convinced to drug Marc with sedatives from Hayley’s stash. However, the situation spirals as playful children interfere, shuffling guests' seating arrangements. Consequently, Jack finds himself next to Amanda and Chaz, while Bryan, unaware, consumes the spiked champagne and begins to lose control.
In the midst of this tangled web, Jack’s attempts to connect with Dina are thwarted by Sidney’s persistent company, requiring Jack to make explicit requests for solitude. As Bryan becomes further disoriented, he embarrassingly converses with Vitelli, only to be outdone by Chaz's confrontation over Jack's feelings toward Amanda, leading to a surprising revelation about Jack and Amanda's tumultuous relationship.
Amidst all this, Hayley is desperate for Jack to keep Marc in check. Jack tries to maneuver through the chaos, but ends up finding Marc awake and cognizant. Together with Sidney, Jack shoves Marc into a closet, effectively barricading him from causing more trouble. At the reception, Bryan is called upon to make a speech, which he struggles through in slurred confusion, humorously mentioning Vitelli in incoherent fragments.
The tension reaches its peak when Marc's affair with Hayley comes to light, igniting Hayley’s fury as she confronts him publicly, only for Roberto to deliver a crushing response. The tumult leads to Roberto's dramatic fall, leaving everyone in shock as The Oracle returns, hinting at alternative realities influenced by the way guests were seated.
In the aftermath of the whirlwind events, Jack inadvertently drinks the drug-laced champagne, creating a frenzy as he tries to dispel its effects, all while Dina recounts her mother’s passing, a moment he can’t fully grasp due to his state. Caught in a cycle of exhaustion, Jack struggles to engage as Amanda tries to confide about her relationship with Chaz, finding him asleep once again.
With all eyes on him, Bryan helps Jack and offers crucial support, encouraging him to be a better friend and brother. As the chaos continues, Marc makes a surprising decision to congratulate Hayley and Roberto, a moment met with gratitude, paving the way for a heartfelt exchange between him and Hayley. Meanwhile, in another corner, Chaz and Amanda part ways without malice, revealing understanding among them.
In a moment of vulnerability, Jack seeks out Dina, offers an apology, and bravely expresses his true feelings. However, their reunion is complicated by her work obligations, leading to a bittersweet farewell. Just when it seems Jack finds a semblance of closure, Vitelli offers him a glimmer of hope for a potential role in his next film.
The film culminates in a kiss between Jack and Dina, celebrating a moment of connection amid the chaos as the credits roll, showcasing a series of humorous "what if" scenarios throughout the wedding reception, reminding us of love's unpredictable nature and the connections that bind us all.
