Alice Clark, a relationship therapist with an elite client list is murdered. Barret Hawk, a expensive fixer was at the scene and ransacked her car and her home searching for something.
The investigation uncovers that Alice was in possession of a priceless manmade diamond. Esposito and Ryan are almost killed in a gun battle with men in clown masks as they try to get the diamond back to the precinct
Turns out Alice and her boyfriend were trying to expose that manmade diamonds of such perfection exist in order to stop the blood diamond trade.
Alice help steal the man made diamond from a client who could lose their fortune due to a contract with a diamond distributor if it came out that they made it. The wife ends up killing Alice to keep their secret.
Kate asks Castle to move his huge lion photo from the bedroom as it she feels it glares at her. He explains he bought Linus the Lion with his first royalty check and it's part of his territory.
In the end, Castle takes down Linus and puts up a collage of seashells in his place. Kate recognizes them as the shells they collected on their first walk on the beach together in the Hamptons. Castle tells her the loft is full of things that are about his story. They need to start to fill it with parts of their story.