Nate and Sophie take cover behind some crates at an outdoor pool as gunshots ring out around them. Sophie complains that Nate should have just gone to the movies because now someone is trying to kill them, and he points out that if he had then she would have been in jail.
Nate and Sophie are at headquarters when Nate receives word that the rest of the team is staying over in DC. Sophie suggests that he go to a murder movie festival and he seizes on the idea until she tells him that she only got one ticket because she has to do some work at her theater.
The next day, Sophie actually goes to the manor of a deceased millionaire, Jonas Gault. The family is holding an auction for his collection of rare Mettier paintings. Sophie talks with Jonas' personal art curator, Katrina Hardt, and they watch as Jonas' daughter Fiona argues with Eleanor, Jonas' fourth ex-wife. Jonas' estranged son Woodman comes over and talks to Sophie, explaining that after years apart he returned in the last year to make amends with his father. As the manservant, Paolo, gets Sophie a fresh drink, Woodman swears that Sophie looks familiar. Much to Sophie's surprise, Nate steps out of the crowd and confirms her (fake) identity. Once they're alone, Nate wonders why she lied to him and came to the auction. Sophie claims she's a Mettier fan and asks how he tracked her down, and Nate refuses to answer her.
The auction is already over and the crowd gathers for a few last words. Katrina thanks Fiona and Jonas' lawyer, Reinhold Shecter, for helping arrange the auction. Sophie tells Nate that there's nothing to stay for... just as Katrina announces that they will show Mettier's first painting, the previously unseen Ma Mystere. Nate isn't surprised when Sophie produces a ticket and they go down to the storage vault. When Nate wonders why she's interested in that particular painting, Sophie insists that it's personal. Meanwhile, Katrina tells the elite crowd that Jonas was the only one who ever looked at the painting within the locked vault. However, when Shecter opens it, they discover that the painting is gone.
Nate grabs Sophie and drags her out, figuring that she'll be the primary suspect. She insists that she's innocent but Nate doesn't believe her. As they get to the door, they find Jim Sterling of Interpol waiting for them. He tells Sophie to hand over the painting and has his agents arrest them. When Nate objects, his ex-partner explains that he's created an Interpol department to prevent stolen paintings from being used to finance terrorism. Nate says that it's just a coincidence that they're there but Sterling doesn't believe it and tells them to turn over the painting. Sophie and Nate can't turn it over and Nate points out that he has no evidence to hold them. Despite that, Sterling handcuffs them and tells Nate that he's paying off his debt to him by looking for evidence before taking him in.
Once they're alone, Nate and Sophie easily free themselves and Nate asks what her connection is to the robbery. Sophie says that the theft was an amateur job and far below her standards, and Nate figures that they have to find the real thief. They start with Shecter down at the vault, claiming that they're undercover Interpol agents. He tells them about the security key pad on the vault and the tag on the frame that would have sounded an alarm if anyone moved the painting. When Nate asks about the insurance settlement, Shecter tells them that Jonas disowned Woodman so Fiona and Eleanor would split the payment.
After the lawyer leaves, Nate examines the vault and realizes it would have taken two days for someone to break through the door. He remembers earlier that Shecter spilled a drink and his hand was sticky, and applies face powder to the keypad. As Sophie complains that Nate doesn't trust her, he discovers that the keys that are the most worn down are not the sticky ones, meaning that someone else recently changed the combination since Shecter used a different combination.
Next the duo go to Jonas' bedroom, smooth-talking their way past Paolo by claiming Sophie was the millionaire's favorite when she was 16. Once they're alone, Nate checks the printer buffer and discovers that Jonas' last fax was the vault access times from the security company. On the day of Jonas' death, there were six unaccounted-for entries and exits into the vault. They figure that Jonas realized someone was breaking in and changed the combination, and was murdered as a result.
Nate and Sophie find Sterling and tell him what they've uncovered. They've checked Jonas' prescriptions and pill supplies and discovered that dozens of pills disappeared on the day of his death. Nate goes to the outdoor drainage pipe from the manor and removes the filter. Just as he expected, inside are the empty gel caps that the killer replaced with poison. Sterling figures that it must have been an inside job and gives Nate and Sophie an hour to find the killer or he'll finish arresting them.
Sophie talks to Katrina, who explains that she came to the manor every day, including the day he died. She was downstairs arguing with Fiona when they heard the maid's scream as she found Jonas dead in his bed. Sophie confirms that they were arguing because Fiona was going to fire Katrina as an unnecessary expense.
That leads to Fiona, and Sophie learns that she was dealing with Eleanor on the day of Jonas' death. Fiona wanted to make sure that Eleanor didn't get any of Jonas' money when he died. That takes Sophie to Eleanor, who tells the grifter that she and Jonas had grown closer after the divorce and were going to remarry. Shecter was writing up the paperwork for the marriage and told her that Jonas wanted the arrangement secret. When Sophie talks to Shecter, he denies Eleanor's claim about marriage and tells her that Jonas planned to put Woodman back into his will. However, Woodman didn't want him. Nate tracks down Woodman at the pool, and he explains that he messed up his life while studying in Paris and his father kicked him out. Woodman started a contracting company and took a job working on the pool just to be closer to Jonas. Bringing the testimony full circle, Woodman tells Nate that Katrina was near Jonas' bedroom right before he died.
Nate and Sophie meet with Sterling and try to work out who killed Jonas. Nate figures that Shecter was embezzling from Jonas which is why he knew what he knew, leaving Woodman as the only suspect without a motive. However, they all alibi each other during the time that the vault was open. Nate gets an idea and takes them to Paolo's room, and reveals that the missing painting is hidden beneath the manservant's bed. Sterling has his people take Paolo away while Katrina takes the painting down and shows it to the audience. As an expert she informs everyone that the painting is real and Sterling closes the investigation. However, Sophie tells Nate that the painting isn't the real Merrier.
Sophie and Nate go to the stable and talk privately. She asks Nate to trust her but he's still skeptical and she offers to prove it. However, Nate realizes that she wants to steal Ma Mystere and one of the paintings that was sold at the auction earlier so she can compare them. He points out that it's a bad idea and Sophie says she doesn't have a choice if he won't trust her. Nate gives in and figures that they'll have to go through Sterling, who has Ma Mystere in the study. However, he tells Sophie that he can outmaneuver Sterling and Sophie goes to steal the other painting.
Sterling's agents are crating up Ma Mystere when Nate comes in and looks around skeptically, and then hints that Sterling hasn't solved the case. Sterling seizes on the hints and figures that Nate won't tell him what he knows because he wants to see Sterling mess up. He tells the agents to secure the painting and then go back to the beginning and redo the investigation.
Katrina is shipping the transport of the paintings to the security van. When she steps away, Sophie approaches the armed guard, Frank and claims that one of the paintings belongs to her. He refuses to turn it over so Sophie goes over to Katrina and whispers that Frank won't give her his company's contact information so she can hire them later. Katrina calls to Frank to tell him to let Sophie have it, and Sophie goes over and collects the painting.
Sterling and his agents search Paolo's room while Nate gives him hot and cold hints. Once he leaves, Sterling has one of his agents follow Nate. Nate ducks the agent by starting a smoke fire in the kitchen and then goes back to the study and switches the crated Ma Mystere for a painting that Sophie has stolen and is keeping outside. The Interpol agent comes in seconds later and Nate tells her that he's trying to find his phone.
Nate and Sophie take the paintings back to the stable and Nate prepares to do a crude gas analysis with household chemicals and a UV light. He discovers that they match but Sophie insists that they can't. Nate wonders why she thinks Ma Mystere is a forgery but she refuses to explain. He admits that if she is right then there is one other possibility: both paintings are forgery. Since the second painting came from Gault's collection and the odds are small that she would have stolen the only other forgery, they realize that all of Gault's Merriers must be forgeries.
The duo goes to Sterling with what they have and he considers arresting them even though he has nothing to charge them with. Nate explains that Jonas was half-blind and the forger could have worked for months replacing the real paintings with fakes. He figures that Katrina is the only one who has the knowledge to forge a Merrier and the position where she could have verified the provenance. Sterling goes out and sarcastically asks if anyone spent months replacing the paintings with forgeries. Katrina immediately runs, much to Sterling's surprise, and they give choice. They hear a scream and when they get to the pool they discover that Katrina has apparently slipped and hit her head on the edge of the pool, killing herself.
As Sterling has his team clean up, he figures that the case is closed and is happy that he now knows where Sophie and Nate working. Once he leaves, Nate and Sophie head back to the brew pub and share a drink. They admit that it was like old time when Nate used to chase Sophie, and she wishes that she had been able to see the painting. However, she won't tell Nate why it means so much to her. Sophie thanks Nate for her trust in him and wonders how he knew she was going to the art auction. He explains that he heard the sound of her heels on the floor as she left and realized she was wearing fancy shows with low heels for running. Once he knew she was going to a party, he called all of the high-end caterers in the area and learned about the art auction. As Sophie goes to change, Nate suddenly gets an idea. When she comes back, she discovers that Nate is gone.
Nate goes back to the manor and studies the vault again. He realizes that Katrina couldn't have cracked the second alarm and gotten to the Merrier. Katrina's hidden accomplice draws a gun on him: It's Woodman, who assures Nate that they're alone and no one will hear him or the gunshot. Nate figures that when Woodman was in Paris, he attended an art school and learned enough to be able to paint forgeries. Woodman came home to scam money from his father and then seduced Katrina and convinced her to help him. He killed her when she panicked and ran, hitting her with a brick and making it look like an accidental fall. They slowly switched the paintings out over the last year, and Woodman finally saw Jonas enter the combination on the special vault. He figured that since no one had ever seen Ma Mystere, he could make a close forgery and substitute it. However, Jonas saw Woodman give the fake to Katrina in the garden and realized that something was going on. He checked the records and then changed the combination to the vault.
As Woodman prepares to kill Nate, he tells him that he isn't so smart. Nate says that he's smart enough to kill his partner and Sophie charges in and hits Woodman with a vase and then runs with Nate.
Nate and Sophie take cover behind some crates at the outdoor pool as Woodman shoots at them. Sophie complains that Nate should have just gone to the movies because now someone is trying to kill them, and he points out that if he had then she would have been in jail. Nate manages to grab Woodman when he comes after them, handcuffs him to a crate using Sterling's handcuffs, and shoves Woodman and the crate into the pool. When Sophie comes back, he admits that he's not sure if he shoved him into the shallow end or not.
They call Sterling and assure him that they've captured all of the guilty parties. Nate tells his ex-partner that Jones changed the code after Woodman removed the Ma Mystere but before he could swap the forgery in. When the theft was revealed after the auction, Woodman used the fake to frame Paolo for the theft. Sterling figures that he can get the real painting's location from Woodman eventually. As he leaves, he tells Sterling that he's always welcome to join him at Interpol and can even bring Sophie with him.
Nate and Sophie go down to the vault and study it carefully. Sophie again wishes that she could have seen Ma Mystere and Nate realizes what happened to the painting. He tells Sophie that Woodman was a contractor and did what he does best. Nate then picks up a crowbar and smashes the fake wall that Woodman used to conceal the real painting inside the vault. The Merrier is a nude of Sophie, and she tells Nate that the artist promised her that no one would see it until after she died. Sophie felt obliged to steal it and fulfill his promise. Nate collects the painting and assures her that she looks better than she did then as they leave.