Harry Deane [Colin Firth] is a British art curator who decides to seek revenge on his abusive boss Lionel Shahbandar [Alan Rickman] by orchestrating a high-stakes con: to acquire a counterfeit Monet, titled Haystacks at Dusk, to sit alongside the real Haystacks at Dawn he already owns. He teams up with a master art forger, the Major [Tom Courtenay], and travels to Alpine, Texas, to recruit PJ Puznowski [Cameron Diaz], the rodeo queen and granddaughter of the sergeant who captured Hermann Göring in 1945.
Harry explains that PJ's participation will lend authenticity since the Monet was last seen at Carinhall after being plundered by the Nazis. She agrees, and the trio drive out to PJ's grandmother's mobile home out in the desert. They hang the fake Monet on the wall and take a picture with the painting in the background, planning to publish the image in a magazine tied to Shahbandar's media empire to spark interest in a forthcoming article about the rodeo queen.
Back in London, Harry meets with Shahbandar and discusses the photos of PJ and her grandmother, turning the attention to the painting. Shahbandar notes that it is a reproduction, pointing out that it hangs in a mobile home in Texas. Harry suggests they at least verify the painting's authenticity, since a true Monet of this stature is rarely reproduced. Shahbandar reluctantly agrees, and Harry begins a frantic search for PJ to follow up on the matter.
PJ offers the painting to Shahbandar for £12 million, but only after arranging to meet him at the Savoy Hotel, a meeting Harry can barely finance. His frustration with PJ grows when she accepts romantic advances from Shahbandar. While they are alone, Shahbandar tells her that art curator Martin Zaidenweber [Stanley Tucci] will replace Harry. PJ then meets Zaidenweber and tells him that the painting has hung in her house since she was a child. When Shahbandar opines that Harry is an idiot, Zaidenweber counters that he is a good man with simply a bad eye for art. Back at the hotel, PJ tells Harry that she no longer wants any part in his plan.
The following night, PJ has dinner with Shahbandar again. They are stopped by his rival, Akira Takagawa, who has long-held a grudge after losing to Shahbandar in the 1992 auction for the first Haystacks painting. By the time she leaves, Harry has called Zaidenweber and convinced him to abandon his allegiance to Shahbandar.
Meeting Harry in the courtyard, PJ agrees to take part in the plan once more. When they arrive at Shahbandar's masquerade ball, Harry heads to the gallery, counting on her to distract Shahbandar while he completes the final preparations.
As the inspection is about to begin, Zaidenweber arrives, having lied to Harry and remained loyal to Shahbandar. He announces that the painting is real, but suddenly, Harry speaks up and disagrees. To everyone's shock, Harry wipes away paint to reveal the likeness of Queen Elizabeth II underneath. Shahbandar declares that PJ will be of no further use to him and tries to re-hire Harry. Harry turns him down, noting his disapproval of the way Shahbandar has treated PJ.
Harry and the Major meet with Takagawa and his men. It is revealed that the Major had painted copies for both Dusk and Dawn paintings. Harry had removed the real Dawn painting and swapped it with its copy. Takagawa tells Harry that the payment for the real Haystacks at Dawn by Monet, £10 million, has been transferred to his Swiss bank account. Harry and the Major thank Takagawa and head on their way.
Meanwhile, while PJ goes through security, she finds a painting from Harry as a gift. She smiles, just before boarding the plane. The end of the film shows Harry and the Major walking through the airport, talking about Donald Trump’s fascination with Picasso.
