While investigating the mysterious shooting death of a young woman, Castle and Beckett face a cop’s worst nightmare when they discover that the gun used in the killing is Det. Ryan’s old service weapon – the one that was stolen by the serial killer, 3XK. Now with the clock ticking, the team must work to solve the murder, track down 3XK and retrieve Ryan’s gun before the killer strikes again.

Rick is asking Kate if she remembers anything about the day she got shot, but she says it’s a blank. They’re investigating a victim who was shot with a 9mm gun. Rick says it looks like a mob hit. (She was shot execution style and buried in cement.) Victim is Jane Herzfeld. Ryan has the bullets in ballistics and Esposito is looking for the next of kin. The victim was tutoring other students, but she canceled that morning. The victim was clutching a cross necklace and it appears that she knew her killer. 

Kate and Rick are asking her mother about her death. They told her about the traces of cocaine on her body. She claimed Jane had a boyfriend she didn’t like, Finn McQueen and he works in construction.

Ryan and Esposito are asking Finn questions and Kate received a call about the ballistics on the gun. The bullett is from a Glock 17 that was in their system. It’s a match to Ryan’s old service weapon. 

It was said that Jerry Tyson snagged Ryan’s weapon and killed Jane. The 3XK killer likes to change things up and Ryan is blaming himself. Ryan is talking to Jane’s roommate and she said six weeks ago, things changed between Jane and her boyfriend. 

Ryan and Kate trace a call that Jane has gotten to a bar and the owner told them that he’s seen Jane before. He said that she was dating a regular customer named Seth. Ryan is thinking that Seth is a patsy. Seth walked into the bar and then tried to run when Kate confronts him. He’s stopped by Esposito and cuffed by Ryan. They find a Glock 17 in his pocket. He’s part of the narcotics division! He tells them to put him in the car before his cover is blown.

They’re questioning him, thinking that he set Jane up. He calls Ryan out for not having control of his own weapon. They start fighting and Kate breaks it up.

The cops went to a Chinese restaurant and asked the owners questions about the case. He claimed she was an informant. They claimed to be at home during the murder and invited the detectives to talk to his household staff to verify his claims.

Rick brought home a box of files and Alexis is beefing up her resume in order to apply for an Ivy League School. Esposito found a locker that held a bag with men’s clothes and two tickets to Phoenix in it. They pulled security footage of Finn arguing with Jane. 

Kate is confronting Finn about Jane leaving town with someone else. Ballistics shows the gun is a Glock 17, but it’s not Ryan’s gun. Rick is coming in with an idea of who the killer is. He thinks that Jin Hyee Chang had the gun and got it from his sworn enemy, Jerry Tyson. Jerry wanted his payback. 

Ryan and Beckett are questioning Phillip Lee with his attorney. Ryan told him he will not rest until he carries the weight of murder. Ryan is taking the rest of the day off, per the Captain’s instructions. Ryan tells Esposito that he’s taking a run at Ben Lee, but they can’t go in there looking like cops.

Esposito approaches the library where Ben Lee is study, looking like a gang banger. He distracts the security guard, while Ryan talks to him. Ryan tells him that Jane died fighting his battle and stuffed her photo in his jacket.

He’s wearing a wire and the cops are stationed outside, listening in. Ben is talking to Phillip about Jane’s death. He’s mad because Jane is dead and he feels like a child. He wanted to know if Jane cried and asked for him. Phillip tells him they’ll talk about it tomorrow. Ben says that if he doesn’t talk to him about it, they can no longer be brothers. Phillip points a gun at him and asks if he’s wearing a wire. The cops storm in and Ben pulls a gun on Phillip, telling him that he has ruined his life. The cops shoot Ben.

Kate confronts Phillip with the stolen Glock 17, which was used to kill Jane. Phillip tells the cops that he has info on 3XK and where he was going and what name he was going to use. Rick is telling Kate that Phillip has bad info because Jerry Tyson knew he would get caught for using the gun.

It turns out that Ben Lee was only faking dead and he’s now entering into the witness protection program. They’re all toasting Kevin Ryan. He makes a toast to bravery and commitment, love and sacrifice, to Jane.