A young street punk who went by the name Glitch has been shot to death in an alley. Also found at the crime scene are three heads in a duffle bag. Ethan Slaughter (guest star Adam Baldwin) is the tough homicide cop who catches the case. One way to describe Slaughter is to say he's Beckett's exact opposite. That doesn't stop Castle from believing he'd make a pretty cool male muse. That's why he teams with Slaughter while his real partner is busy prepping for a trial. Once again, Beckett senses Castle is pulling away.
Slaughter doesn't sugarcoat the news of Glitch's death to his father, Brian Reilly. The guy happens to be a leg-breaker for the Westies. He's likely to take the law into his own hands when it comes to avenging his son's death. Slaughter intends to be there when he does this. In the meantime, Castle asks Esposito to help ID the heads found in the duffle bag. No problem. All Javy needs is front row Knicks tickets for this particular favor.
Slaughter gives Castle a gun as they storm a bar where Brian Reilly is seeking vengeance. Castle nervously barks, "NYPD! Associate Civilian Investigator. Nobody move!" With that, the place clears out. Slaughter arrives in time to grab up Brian. Shea, the man he was going to shoot, tries to escape. Thankfully, Slaughter's temporary partner is there to take him down. It isn't pretty, but Castle gets the job done.
In relationship news, Beckett vents to her shrink about her situation with Castle. She's wondering if she waited too long to tell her partner how she feels. As for Castle, he's been hiding incoming college acceptance/rejection letters from Alexis. He's trying to protect her from being disappointed on a daily basis. He's also protecting her from Slaughter, who makes some indecent remarks about his little girl. So Castle decks him. Slaughter is a truly scary character, but he now knows not to mess with Castle when it comes to his daughter.
The heads in Glitch's duffle bag belonged to members of the Jamaican gang currently in a turf war with the Westies. Castle believes the victim may have killed them to make amends for messing up a job for his gang. He also believes his new partner may be crazy after he thinks Slaughter shot an informant. He didn't. It was just a test to see if Castle could be trusted. The men have a laugh about it later, but that doesn't mean Castle is cool with the violent way Slaughter gets info from others.
A Mexican gang led by a bad man named Cesar Vales is looking to set up shop in New York. Castle believes he may have been trying to start a war between the Westies and the Jamaicans to clear the way for his operation. Ryan and Esposito realize that Slaughter and Castle are going to try to take down Vales. That's something that could get their friend killed. That's why they trace Castle's cell phone to provide some much-needed backup before anyone gets hurt.
Esposito and Ryan are worried that Castle is getting in too deep with Slaughter, who kidnaps a member of the Vales gang. He believes the kid is a weak link who will crack under pressure. Castle realizes that Slaughter is coercing this guy into flipping on his boss. In other words, he gets the kid to lie so that he can make the bust. Castle knows Slaughter crossed the line. He wants no part of it. He needs help. He needs his partner. His "real" partner. As it turns out, Beckett has been helping him from afar all along. All this time, she had his back.
Beckett shows Castle a traffic cam picture that shows Vales' car in a different location at the time of the murder. It's not enough to prove he's not the killer, but it's a start. Castle and Beckett dig a little deeper. This ticks off Slaughter, who could get Beckett in a lot of trouble for interfering in his case. A reevaluation of the probable events that went down at the crime scene lead Castle and Beckett to real killer-Glitch's father, Brian Reilly.
After Cesar Vales is cleared of Glitch's murder, Beckett warns him to get out of her city. Slaughter is totally turned on by Beckett's admonition. He's also totally ticked at Castle for not having his back. That's why he slugs him in the gut before heading out of the precinct. Castle may be winded, but he's able suck it up long enough to thank Beckett for helping him. She says, "No problem, Castle. That's what partners are supposed to do."
The news that Alexis has been accepted to Stanford is bittersweet. After all, they rejected her initially. How is she supposed to get over something like that? Castle says, "I guess the question is... do you want it badly enough to get over being hurt?" Somehow we get the feeling he's asking that same question to himself, don't you?