Kat Ellis, a single New Yorker, returns to her parents' home in London to serve as the maid of honor for her younger half-sister Amy's wedding. Little does she know that the best man at the event will be her former fiancé, Jeffrey, who unexpectedly ended their relationship two years prior. Eager to impress him and perhaps stir a bit of jealousy, Kat enlists the help of the charming escort Nick Mercer to act as her boyfriend.
As Kat attempts to make Jeffrey envious, her scheme quickly spirals out of control when Nick effortlessly convinces everyone, including Kat herself, that they are profoundly in love. In an unexpected turn of events, Kat finds herself genuinely falling for Nick, who reciprocates these feelings.
However, on the eve of the wedding, everything changes when Kat learns that Amy had an affair with Jeffrey during their previous relationship—a revelation that shatters her trust. Nick, having known this information a day earlier, unintentionally leaves Kat feeling betrayed by both him and her sister. Heartbroken, she sends Nick away, allowing him to return to America and leaving behind the money she had originally paid him.
On the day of the wedding, Kat's step-father observes her distress and questions whether Nick might be “the one” for her. This prompts a realization in Kat about her true feelings for Nick, sparking a desperate search to find him. Meanwhile, during a moment of honesty, Amy confesses her infidelity to her fiancé, Ed, while professing her love for him.
This revelation sends Ed into a frenzy, leading him to confront Jeffrey and accuse him of being a "back-stabbing weasel," even as Jeffrey defends his actions based on the timeline of his relationship with Amy. In a heated exchange, Ed reminds Jeffrey of his engagement to Kat, illustrating the real wrongdoing that has transpired.
As events unfold, Nick finds Ed and the two have a deep conversation about love. Ed realizes his love for Amy outweighs his anger and receives sound advice from Nick about pursuing reconciliation. They make their way back to the church, with Nick stepping in as the unexpected new best man. Just before the ceremony, Nick shares with Kat that he'd “... rather fight with you than make love with anyone else,” culminating in a passionate kiss that signifies their newfound commitment.
Kat and Nick embark on a genuine relationship, leading him to abandon his former job. Meanwhile, Amy and Kat begin to mend their relationship. After the wedding, a light-hearted moment occurs between Kat's cousin, TJ, and Woody, hinting at budding romance. In stark contrast, Jeffrey remains blissfully ignorant, ending the story still vying for the attention of a neighbor, learning nothing from his past mistakes.
