After the commercials, Beckett is helping bring in the fan mail, in several mail bins. Castle is in with the police captain, who calls her in to his office to tell her that Castle has volunteered to assist, and he thinks it's a good idea. Beckett is floored. She asks to talk to the captain in private, but he says no. That's cold. Beckett and Castle go through the fan mail, and Castle plays peek-a-boo with her over a letter. Beckett asks why he's here, and Castle says for the story -- he wants to know why these two people, why these methods. She says there isn't always a story, so Castle breaks her down, dissecting why she became a cop and not a lawyer, figuring out that something happened to somebody she cared about, and the perp was never caught. She doesn't deny it, but she does find a crayon drawing of the flower crime scene in one letter. So their perp is five. Put out an APB&J.
In the squad room, we find out there are prints on the drawing, but they have to wait for the lab to clear its workload before it can get to them. For a serial killer? They have to wait a week or so? I doubt that, and so does Castle, who calls the mayor and shoots the breeze with him about a party they were both at before asking if he can prioritize their prints. Done and done. The detectives are amazed, and Beckett tells him he can't just cut the line. Then word comes in of a third body, and calling the mayor seems like it was a really, really good idea.
A woman in a yellow dress floats in an indoor swimming pool. It's a scene straight out of the book Death of a Prom Queen, apparently, and Beckett tells Castle to stay put. But the minute the body is out of the water, he immediately walks over to the medical examiner, Lanie, who introduces herself as a big fan. (Although she didn't recognize the flower crime scene earlier, so how big a fan can she be?) He correctly discerns that the knife was inserted post-mortem, and that she didn't drown. Lanie is impressed, and Beckett is ticked, but then word comes in on a match for the prints: Kyle Cabot of Brooklyn. They haul ass over to the suspect's building, and Castle is told to wait in the car while they go in. He promises, Scout's honor.
Inside the apartment, they find scary drawings like the one they found in Castle's mail, plus several Castle books, a picture of Castle, and a picture of Ms. Tisdale. They're looking at the guy's Wall of Weirdness when Castle shows up and says "Oh, that's creepy!" Apparently, he was never a Scout. They've just discovered the gun that killed Alison when they hear a thump, and open a closet door to find Kyle, huddled in a corner, scared and clearly out of it. Back at the station, Beckett reveals that Kyle has a history of pervasive developmental disorder and delusions, and Alison Tisdale was his caseworker. The other two ate at the diner Kyle worked at. Open and shut case.