Linus, the big lion featured in a huge portrait hanging in Castle's bedroom, thoroughly freaks out Beckett on a daily basis. She'd like Castle to move it to another room. This request touches a nerve. Castle purchased Linus with his first royalty check. The great cat debate gets put on hold when famed relationship expert Alice Clark is found dead in her car. Castle knows of her book about mating rituals. The author believes we can improve our relationships by acknowledging our animal nature. As for her murder, the killer was looking for something judging by how her car was torn apart. Her phone has a reminder alarm displaying the code: NF1945.
Esposito discovers that Alice's apartment was ransacked. The building security video gives a partial shot of a possible suspect. Alice's boyfriend, Matt, doesn't recognize the guy. He says Alice sometimes had difficulty with aggressive clients. She seemed a little rattled at lunch the other day. Castle believes secrets she may have held for her clients may have led to her ultimate demise. Ryan discovers that the code on her phone is the tail number of a private jet Alice chartered. She told the pilot to be on standby to go to an unknown destination.
Ryan and Esposito track down a suspect in a van who wipes out some computer files and shreds a photo of Alice before he's apprehended. The guy turns out to be Barrett Hawk, New York's top fixer. He's been sweeping the dirt of the rich and famous under the rug for over a decade. The guy is very careful so charges never stick to him. In other news, one of the victim's client files is missing. It's for Andrew Spenser, a hotel and real estate tycoon who was divorcing his wife, Monica. The guy is worth billions. Those files could cost him a lot of money in a settlement.
Andrew Spenser seems genuinely shocked that Alice is dead. He says the victim turned over his file voluntarily for a favor. She wanted a room in one of his hotels that would assure her anonymity. She was keeping a huge diamond worth millions in the hotel safe. Castle believes the story behind the gem may help solve the case. An expert says the diamond is worth at least $60 million. Esposito and Ryan are on their way to bring the valuable stone back to the precinct when… CRASH! A large vehicle smashes into their car. Armed gunmen in clown masks fire on them. Espo and Ryan return fire. The bad guys flee. The diamond is safe… for now.
Castle believes the case may have something to do with the victim having spent time in the diamond-rich area of the Congo. Beckett follows another lead in Steve Warner. He was one of Alice's clients who had connections in South Africa. His wife, Janet, was photographed at an event where the diamond is seen around her neck. The couple claims the photo shows nothing more than costume jewelry. Still, they met with Barrett Hawk just before Alice was killed. Castle says, "Looks like New York's famous fixer is in a bit of a fix."
Hawk tells Beckett that the Warners hired him to find the diamond, but Alice was already dead when he got to her. He says his employers were mugged while walking home from a benefit. He knew it was an inside job. Alice knew the diamond was real. She had an accomplice help with the theft. Ryan learns the diamond is man-made. Its value is actually priceless. Castle believes the diamond industry would be devastated if precious gems could be made in a lab. The team learns that a South African criminal named Leo Wyngaard may be involved. He's on the terror watch list. A photo shows that the guy is actually Alice's boyfriend, Matt.
Matt claims he met Alice in Rwanda. They were in love. He had dedicated his life to working against the cartels in these countries. He tells Ryan and Esposito that one of Alice's clients, Steve Warner, was creating manmade diamonds. They needed proof of this, so they stole the diamond. This would put an end to the cartels. Steve Warner admits that he hired Hawk, but only because a dangerous cartel would find out. As it turns out, it was Janet Warner who killed Alice because she was going to expose them. Case closed.
At the end of a long day, Castle shows Beckett a surprise in his bedroom. Linus is gone from his wall. In his place is a frame featuring seashells that Castle and Beckett collected during their first walk in the Hamptons. This idea came from a section in one of Alice Clark's books. Castle's place is filled with items telling his stories, but the shells represent Beckett, too. It's their story. Nice.