As Castle pets a chocolate Lab outside the latest crime scene, Beckett arrives and they head into an art gallery where the owner, Victor Fink, has been shot dead. Fink's assistant, Darius Langley, says the gallery was closed and there was no forced entry. Ryan notices there are two bullets in the body, two in the wall, but five shell casings on the ground. So where's the fifth bullet?
A mugging victim found near the murder scene may have information on the case. The only problem is that he has amnesia. It's always something, right? The guy's big reader though, meaning he reads thick books. It's a good thing, too. The copy of Crime and Punishment he had on him holds the fifth bullet lodged inside its pages. Castle asks the guy to sign something, which calls upon his unaffected procedural memory. When the only part of his signature they can read is the first letter in his name, they decide to call him "Jay."
Rocco Jones, Fink's assistant before Darius, says he was making forgeries of high-end paintings. The fakes were sold to Bahir Harun, an attaché who has full diplomatic immunity. When Bahir Harun comes in voluntarily, he explains that he knew the paintings were frauds. He bought them to sell to clients who wanted to impress their wives without paying top dollar. Harun admits he met Fink to pick up a painting the night of the murder, but left when he saw Fink arguing with another man. Guess who? Yep, it was our favorite amnesiac, Jay.
Castle realizes the chocolate Labrador he was petting outside the crime scene is Jay's dog. That's why Jay had the plastic bag in his pockets. The world needs more conscientious pooper scoopers like this guy. They find the Labrador at Animal Control and the dog tag leads us to Jay's real name: Jeremy Preswick.
They find his ex-wife, Emma Carnes. Jeremy doesn't remember her, but cannot stop commenting on how everything must have been his fault since she's so nice. Seeing this side of the man she once married, Emma starts to feel for him all over again. Emma accompanies Castle, Beckett, and Jeremy back to Jeremy's apartment. To everyone's surprise they find a gun with five bullets missing. It looks like Jeremy is their killer. Too bad. We kinda liked the guy.
It's obvious that Castle and Beckett are fond of Jeremy, too. While digging for more clues, they find Emma collecting supplies for Jeremy's dog at his apartment. Emma comments on a painting they bought together, a Taglia. The one in the apartment is a fake because it doesn't have a thumbprint that Jeremy accidentally left on it. So where's the original?
Beckett and Castle find Bahir Harun packing a painting-sized portfolio in the trunk of a double-parked limo. Since the portfolio is now in the car, diplomatic immunity no longer applies. They open the portfolio and find the missing Taglia. Harun says he bought the painting from Fink's assistant, Darius, who contacted him after the murder and sold him the painting.
Beckett and Castle confront Darius with what they know. Darius has been living under a fake name and already has convictions for counterfeiting. They obtained a signed statement from Jeremy's landlord saying Darius bribed him to let him into Jeremy's apartment twice; the first time to switch the Taglia painting with a forgery and the second time to plant the gun that Darius used to kill Fink. Busted!
A reunited Emma and Jeremy decide to give their relationship a second chance. Jeremy still has amnesia, so talk about a fresh start. Beckett really hopes they make it. Castle says, "Why, Detective Beckett, I had no idea you were a romantic." Beckett responds, "I also sleep with a gun. Bet you didn't know that either."
Speaking of second chances, Martha has been ga-ga over an old high school flame she's been dating. But when things go south, she drowns her sorrows in ice cream with Alexis. Then Martha's man begs for a second chance, but she's hesitant. Maybe she should just settle for those fond memories from high school and not take any chances. Castle tells his mom that it's time to make some new memories. Looks like Beckett isn't the only romantic around these parts. But we're pretty sure Castle doesn't sleep with a gun.