Kat Ellis (Debra Messing) is a single New Yorker who returns to her parents' house in London to be the maid of honor at her younger half sister Amy's (Amy Adams) wedding. The best man is none other than her former fiancé, who unexpectedly dumped her two years ago. Anxious about confronting him and eager to impress him, she hires suave escort Nick Mercer (Dermot Mulroney) to pose as her boyfriend.

Kat intends to make her former flame, Jeffrey (Jeremy Sheffield), jealous, but her plan backfires when Nick convinces everyone, including her, that they are madly in love. Kat then feels herself, too, falling for Nick as he slowly falls for her. The night before the wedding, Kat discovers Amy slept with Jeffrey when they were still together, and that Jeffrey dumped Kat because he believed he was in love with Amy. Nick had discovered this fact a day earlier, and when Kat finds that out, she feels betrayed from all sides, and puts Nick off. He decides to return to America, and leaves Kat the money she had paid him.

On the wedding day, seeing Kat distressed, her step-father (Peter Egan) asks Kat if Nick 'is the guy for you', and Kat realizes he is, so she sets off to find him. Meanwhile, just before the wedding, Amy confesses her betrayal to her fiancé, Ed (Jack Davenport), but professes her love for him. Ed, upset, chases Jeffrey out of the church and down the road. Jeffrey in distress of the chase, said he gave up on Amy and believes he's done nothing wrong. To which Ed, calls him a "back-stabbing weasel", though Jeffrey believes he's still done nothing wrong because he slept with Amy before they dated. Ed shouts out that he was engaged to Kat, proving he was still in the wrong for what he did to Kat. Nick, driving away, picks up Ed as Jeffrey disappears into the woods.

Nick and Ed talk about love, and Ed decides he loves Amy more than he is angry. To make it more clear that he should go back, Nick tells Ed if he went back the couple would end up having great make-up sex. To which, Nick helps urge him more to return to the church, so they end up getting married, with Nick as 'new' best man. Just before the ceremony, Nick tells Kat he realized he'd "... rather fight with you than make love with anyone else", and they kiss passionately. Kat and Nick begin a real relationship together. Amy and Kat now reconcile and Kat lets go of her anger and forgives Amy since she confessed the truth to Ed. TJ (Sarah Parish), Kat's cousin also apparently enjoys a moment with Woody (Jolyon James) after the wedding. Jeffrey, the main cause of all the trouble, learns absolutely nothing. At the end he is seen trying to get the attentions of a female neighbor.