The episode begins with an old man finding a dead body of a young guy. When he opens the dead guy’s bag to check for valuables; he finds severed hands inside the bag. He is struck with terror. At the office, Castle is playing with dolls and Martha walks in. She hands over some letters to him. Castle hides them away; they are acceptance letters for Alexis from Stanford and he will show them to her later. Castle is watching the news of the murder on television, when Martha quietly takes those letters from the drawer. Just then he sees a cop on television and wants to meet that guy. Castle visits Kate and asks her if she knows a cop named Slaughter.
Meanwhile, Slaughter is at the autopsy lab, where the doctor tells him that the COD is the three shots the victim received on his chest. Just then Castle arrives and checking the other dead bodies, he infers that the victim dug the bodies and cut the heads off. Castle tells Slaughter that he wants to ride along with him for a day on this case. At first, Slaughter denies, but later he agrees when Castle tells him that he is friends with the Mayor. Also Castle has to give away his coat to him. Slaughter sets the ground rules and takes Castle with him. They meet a guy named Brian and Slaughter describes the way his boy was killed; he asks Brian if he knows anything about this murder. He does not want to and tells the duo that he needs to make arrangements and leaves.
Slaughter tells Castle that Brian would hunt down the killer himself and they will follow him. Just then Slaughter is gets three names from his colleague and Castle tells Esposito to run them for him. but he denies; but agrees to do it if he is given basketball tickets. Castle agrees. Esposito tells him that t he cop, Slaughter, has already been through three partner; all killed in the line of duty. Next, they find Brian, digging for information. They then see Brian entering a bar and they follow him inside. Slaughter asks Castle if he is carrying a weapon; but Castle isn’t. Slaughter announces rule number three; if Castle wants to ride along with him, he needs to carry a gun and hands one to him. Castle enters from the front and slaughter takes the back door. Brian has his gun drawn to a guy’s head.
Castle pins down the guy when he tries to escape while Slaughter has Brian at his gun point. Castle and Slaughter walk in with two arrests. Castle updates Kate with the incident at the bar. Kate reminds him that he is a writer and not a cop. He is not supposed to be doing this and it isn’t his place. He tells her that he could write an excellent story about Slaughter and his gritty style. Castle and Slaughter are interrogating one of the guys and he tells them that he didn’t kill Glitch. The duo wants to know as to why Brian had his gun on him. He tells them that he hadn’t seen Glitch in two days as a guy named Finn didn’t want him to meet Glitch. Glitch had messed up a job and the only reason that Finn didn’t go after him was Brian. He also tells them that Glitch had told him that he is going to do something to get back in good with the Westies.
Castle gets Esposito’s report and sees that all three heads belonged to the Jamaican gang. So maybe Glitch cut those heads off to please the Westies. Meanwhile, Kate thinks that Castle has moved on and doest want her anymore. Slaughter tells Castle that he needs to give more time and attention to Kate. He also mentions that there is a new red head working in the morgue who he finds sexy. Castle tells him that she is his daughter, and lays a punch on Slaughter’s jaw. Just then Slaughter sees ‘his guy’ and follows him into a building. He asks Castle to wait outside and keep an eye. After a while, Castle hears gun shots from inside the building. Slaughter walks out and tells Castle to pretend that they were never here.
At the office, Castle tells Esposito that Slaughter is insane and Esposito tells him that he still want his tickets. Just then the guy, who Castle thought Slaughter killed earlier, walks into the station. Castle is shocked and both the guys have a hearty laugh. The guy tells him that he is mark Gibson and he is an undercover in the task force. Slaughter tells Castle that he was testing him whether he would have his back if things got ugly. He passed. Mark tells them that they were right about the Jamaican’s taking down Glitch and it was a guy named Maxi who did the shooting. Maxi admits that he did meet Glitch to ambush him but he never fired any shots. He tells them that there is another gang involved; a Mexican gang.
Castle calls Ryan and asks him to run a match on a license plate which begins with RX. But Ryan too wants something in return. Slaughter wants to celebrate Castle’s first day on the job with him. Next day, Castle is lying on his couch; passed out with the celebration. Just then he hears his daughter screaming. She is happy seeing all the acceptance letters from the universities. Castle is back on the case with Slaughter. Slaughter tells him that he looks like crap and offers him a drink. But Castle doesn’t want to. They start discussing the case and Slaughter tells him that he ran the plate himself and that it belonged to a Mexican guy named Vallas. He thinks that the Mexicans are making the Jamaicans and Westies both take out each other and that Vallas is coming to town. At the office, Ryan tells Esposito that Valla is from a high level mafia gang and had killed three cops.
Slaughter and Castle arrive at a scrap yard and declare that they are from NYPD. They talk to Vallas and ask him whether he knows Glitch but Vallas says a no. but Slaughter tells him that he has an eye witness who has seen him and his guys two nights ago, shooting Glitch. Just when things were starting to get ugly; Esposito arrives with a bunch of cops and saves the duo. Slaughter doesn’t thank Esposito as he thinks that he just ruined his play. Slaughter wanted to escalate things so that he could identify the weak man in the gang. Castle looks at the kid. Castle is having coffee at the station when he receives a message from Slaughter asking him to meet him downstairs. Slaughter is driving his car when Castle hears that someone is in the trunk of the car.
Slaughter tells him that it is the weak kid. Just then Slaughter drives the car in circles and the guy in the trunk is screaming. Finally the kid agrees that it his boss Vallas did the killing. They take the kid back to the station. The kid tells them that Vallas wanted to use Glitch and the severed heads to spark a war between the Westies and the Jamaicans. The kid is scared to talk but Slaughter tells him that if he co-operates then could get his brother’s sentence lightened, who is in prison. Castle is listening into the interrogation and realizes that Slaughter has coached the kid to give the statement. Castle confronts him. Slaughter tells him that Vallas is a bad guy and that if he has a chance of putting him behind bars, he is going to take it.
Castle says that he is not in with him for this one. Slaughter is upset and he leaves. Castle asks Kate for help and she tells him that this is not her case. Kate shows him a traffic cam shot which shows that Vallas left the area just before Glitch was killed; so Vallas hasn’t done it. But this isn’t enough to change Slaughter’s mind as Vallas’ face isn’t clear in the picture. They go to the crime scene. Slaughter too arrives at the scene. Kate tells him that he coerced a confession out of that kid. Slaughter tells her that he wants Slaughter behind the bars as he is a bad guy and leaves. Kate and Castle stay back on the scene. They are wondering as to why was Glitch hiding when he could escape. Castle infers that he was waiting for help; the one he had called for using a pay phone nearby.
Next, Kate and Castle tell Slaughter that it was Brian who killed his own son. They found out that Glitch had called Brian’s cell using the pay phone. Brian tells them that this time not only did Glitch screw up but also crossed the line. And he being Glitch’s father, made him responsible for Glitch’s actions. And hence he killed him. But Slaughter is upset that Vallas still walks. Kate meets Vallas and tells him to leave the city or he will have the full force of NYPD crashing down on him and make his life a living hell. Meanwhile, Slaughter launches a punch on Castle’s stomach, for not having his back on this one. He thanks Kate. Back at home, Alexis is having a hard time deciding on the college; and Castle thinks that an ice cream could help her decide. The episode ends.