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.