Grace Bennett is a New York University-bound, Texas high-school student who works as a waitress in a restaurant with her high school dropout, best friend Emma Perkins to earn money for a post-graduation trip to Paris, France. Grace's stepfather pays for her uptight older stepsister Meg Kelly to join them, and Emma goes to Paris despite her boyfriend Owen's marriage proposal.
The trip quickly proves to be a disappointment; the girls discover they were ripped off, with a cramped hotel room and a tour that moves too fast for anyone to appreciate anything properly. After missing the bus and being left behind by their tour guide at the Eiffel Tower, the three girls seek refuge from the rain in a posh hotel. The hotel staff and paparazzi mistake Grace for the spoiled celebutante British heiress Cordelia Winthrop-Scott, Grace's double, who leaves rather than stay to attend an auction for a Romanian charity to which she is to donate a Bvlgari necklace. Despite Meg's protests, the three girls spend the night in Cordelia's suite and fly to Monte Carlo with Cordelia's luggage the next day.
At the Hotel de Paris in Monte Carlo, the girls meet Theo Marchand, the son of the philanthropist hosting the charity. Theo is cold towards the three because he loathes Cordelia's spoiled nature. He escorts them to a ball, where Grace successfully fools Cordelia's aunt Alicia and Emma dances with Prince Domenico. Meg reunites with Riley, an Australian backpacker she briefly met in Paris. They find they have things in common and spend time together before he leaves for Italy. Meanwhile, Owen decides to travel to Paris in search of Emma.
When Grace participates in a polo game, Alicia discovers the impersonation because of Grace's different riding style. After calling the real Cordelia confirms her suspicion, Alicia believes Grace is a lookalike that Cordelia hired to take her place while she parties. However, after Alicia threatens her to expose the impersonation, Grace convinces her not to endanger the charity auction, and Alicia agrees to keep quiet.
Theo grows attracted to "Cordelia's" frank personality, while Domenico invites Emma to a party on a yacht. Emma dresses up for the party in the Bulgari necklace but meets Meg on the way, and Meg takes the necklace for safekeeping but later forgets about it, accidentally leaving it in Riley's backpack. At the party, Emma becomes disillusioned by Domenico's arrogance toward the waitresses.
After seeing Emma in the newspaper account of Grace's appearance at the ball, Owen goes to Monte Carlo. Cordelia also arrives there and sees Grace in the newspaper. Finding the necklace is missing, she calls the police. The girls are in search for Riley after Meg tells Grace and Emma that he is leaving on a train to Italy, but later on he shows up at the hotel with the necklace to return to Meg. They find Cordelia in the suite, and when Cordelia threatens to withdraw the necklace from the auction, the girls panic and tackle her to the couch. When the police come to the door, they muffle her screams, and Grace covers for them. They tie Cordelia to a chair and gag her by stuffing an apple in her mouth, so Grace can take her place at the auction while Emma watches Cordelia. Owen arrives to the hotel in France and finds Emma at the suite, and they reconcile.
Cordelia eventually escapes and reveals Grace's fraud at the auction. She demands Grace's arrest, but after Grace's sincere public confession, Alicia bids the unexpectedly large amount of €6 million for the necklace to save her. When Cordelia still insists on the arrest, the inspector observes that there is legally no crime; Cordelia has no evidence that Grace did anything wrong now that the necklace has been returned, she can't prove when Grace was posing as her without compromising her own recent actions, and Cordelia can't force legal action just because she's upset about the situation. After the auction, the three girls leave and go on their separate ways. Meg decides to join Riley on his global travels around the world while Owen and Emma return to Texas, marry, and move into their own home. Theo and Grace reunite at the Romanian school, which the former runs and the latter volunteer-teaches.
