Mercenary thief Laure Ash (Rebecca Romijn) participates in a diamond heist at the 2001 Cannes Film Festival in France. The plan is for Laure to steal a necklace of valuable diamonds from the ensemble of a female attendant named Veronica (Rie Rasmussen). Posing as a French photojournalist, Laure gains access to the festival gathering, where she lures Veronica to the ladies room and begins to seduce her in a stall, during which Laure's two accomplices "Black Tie" (Eriq Ebouaney) and Racine (Édouard Montrouge), plus a janitor co-conspirator, provide various support; Black Tie, disguised as a security guard, subdues Veronica's bodyguards, then sneaks into the ladies room, where Laure takes the diamonds off Veronica and drops them on the floor, letting him put them in a bag and replace them with glass fakes, while Racine, dressed as another security guard, monitors the events from outside the ladies room. When the deception is discovered, Laure double-crosses her accomplices and escapes to Paris with the diamonds. Black Tie tries to stop her but gets shot by a security guard and arrested.In Paris, Racine pursues Laure and tries to catch her to get her to reveal where she has hidden the diamonds. Through a series of events, Laure escapes to a country house where she is mistaken for her own doppelgänger, a missing Parisian woman named "Lily" (also portrayed by Romijn) who had recently disappeared. While Laure luxuriates in a tub in Lily's parents' home, the real Lily returns and commits suicide while Laure secretly watches, giving Laure the opportunity to take her identity for good and leave to America. On the plane, Laure meets a man seated next to her and introduces herself as Lily.Seven years later, Laure (still in her identity as "Lily") resurfaces in Paris as the wife of Bruce Watts, the new American ambassador to France (Peter Coyote). After arriving in France, a Spanish paparazzo named Nicolas Bardo (Antonio Banderas) takes her picture. The picture is displayed around Paris, and Black Tie (recently paroled from prison) spots Bardo's photo while in the middle of killing Veronica by throwing her into the path of a speeding truck. With Laure exposed to her two vengeful ex-accomplishes, she decides to frame Bardo for her own (staged) kidnapping so she can claim the ransom Bruce will pay for her release and flee Paris once again to begin a new identity. Bardo is further manipulated by Laure into following through with the "kidnapping", and in the process, they begin a sexual relationship.The pair eventually meet with Bruce for an exchange on a bride on the Saine, but Bardo has a crisis of conscience at the last moment and sabotages the scheme instead of killing Bruce. In retaliation, Laure executes both Bardo and Bruce, only to be surprised afterward by her ex-accomplices, who promptly beat her up and throw her off a bridge to her seeming death in the river below....However, in an extended twist ending, the entirety of the movie's events after Laure enters the bathtub in Lily's home rewind and are revealed to be a dream or premonition of the entire seven years. Laure is surprised to find herself back in the bathtub at Lily's parents' house. She spies Lily arriving as before, but this time stops her from committing suicide. Laure gives Lily her plane ticket to America and tells her to build a new life for herself there with Bruce.Seven years later in Paris, Laure and Veronica, who is revealed to have been Laure's partner/lover all along, chat about the success of their diamond caper. All of a sudden, Black Tie and Racine arrive seeking revenge, but in a twist of fate, they are killed by the same truck that killed Veronica in Laure's alternate reality. Bardo, witnessing all these events, introduces himself to Laure, swearing that he has met her before, with Laure replying, "Only in my dreams."