Alexis Castle sits silently in a darkened living room. She's not alone. A mysterious figure approaches in stealth-like fashion. BLURP, BLURP! Alexis is startled by the laser tag gunshots of her father who is dressed in a ridiculous flashing uniform. Castle finally got the drop on his daughter, who believes it's time they hung up the lasers and started acting like adults. These aren't the only growing pains being suffered by our favorite mystery writer. He believes the best way to survive his now-strained relationship with Beckett is by making their next murder case their final one.
David Locke is a currency trader who was murdered in the parking lot of his office. Perlmutter says the victim died of blunt force trauma to the head, but also explains that the bite marks on his arm were from someone trying rip off his flesh. Fibers from a 19th Century outfit are found at the crime scene. There was bad blood between the victim and a coworker, Charlie Coleman. Beckett and Castle find the disturbed man handcuffed to the radiator in his apartment. Charlie claims he was attacked along with Charlie. Their assailant: a zombie!
Charlie's story doesn't sound all that crazy once Esposito confirms that the bite marks on his arm match those found on David Locke. Surveillance video shows a mysterious figure wandering through the streets near the crime scene. The suspect is wearing haggard old-fashioned clothing. Close-ups reveal that his ensemble isn't the only thing looking well-worn. The gaunt, cut-up up face suggest the killer is, indeed, a zombie. Needless to say, Castle is on top of the world.
The good times continue once Castle realizes that Alexis was playing him yesterday when she told him they needed to grow up. It was all part of her master plan to zap her unsuspecting dad with her laser. As for the case, Charlie is cleared but opts to remain in jail until they catch the zombie. The team learns that David was trying to get back together with an ex named Greta, who says her jealous boyfriend, Tom Williams, was missing from bed during the time of the murder. He claims he was at the all night library studying.
A chemical found on the bottom of the victim's shoe indicates he was near an abandoned factory in a dark, desolate alley. Castle and Beckett find two people running for their lives when they check out the scene. They are being pursued by a zombie hoard. No, really! As it turns out, it's just a bunch of people acting out their zombie fantasies. Castle can't believe he didn't know about this fantastic subculture. He also learns that if Beckett could be any supernatural creature, she would choose Van Helsing. Castle feels it's fitting that she would pick a monster slayer.
One of the zombie players recognizes the suspect as Kyle Jennings. The team finds him dead in his apartment. Then the guy is undead. Kyle Jennings freaks out everyone when he sits bolt upright at the morgue. He's eventually hospitalized. The teeth marks and blood on his clothes indicate that Jennings is the killer. The guy has no recollection of what happened the night David Locke died. Castle tells Beckett that people tend to remember life-altering events. Beckett acknowledges this but adds that sometimes people need time to deal with it. Somehow we know they're not talking about the case.
Perlmutter knows why Kyle has no memory of the murder. He was slipped a drug that promotes compliant and suggestible behavior. The product is known as the "zombie drug" on the streets. Castle and Beckett learn that the dealer who supplies this product is a smug Tom Williams, who knows the cops can't prove he had anything to do with the murder. Beckett has no choice but to release him from police custody, but she makes sure he knows that Kyle Jennings is out on bail, too.
Tom Williams wants police protection from Kyle, so Beckett has Esposito drive him home. When they arrive, a violent figure smashes the windshield of the car. It's a zombie! The undead dude quickly neutralizes Esposito. He then goes after a terrified Tom Williams, who admits that he was behind David Locke's murder. That's all the team needs. Esposito pops back up and the zombie backs off. Okay, it isn't really a zombie. It's Castle in disguise. His freaky zombie portrayal was just chilling enough to scare up a confession. It also creeps out Esposito big time.
Kyle is extremely grateful that his name has been cleared. He even lets Castle keep the zombie costume. It should be a memorable keepsake from his final case with his longtime partner. Of course, Castle has second thoughts once Beckett reveals that she's been in therapy. She's almost where she wants to be, which is in a place where she can accept everything that happened the day she was shot. Yes, everything. Castle understands. That's why he says that he'll see her tomorrow. Whew!
Back home, Alexis tries to sneak up on her father with her laser tag gun. She's startled by the zombie seated in her living room. Castle takes advantage of her hesitation to finally win this game they've been playing since Alexis was five. This upsets Alexis, who announces that she'll be going to school locally at Columbia as long as her dad gives her space. She then shoots him with her laser to start a new game before fleeing the room. Castle assures her this isn't over. He then stares straight ahead telling us in his best zombie voice that THIS ISN'T OVER. What a way to leave us hanging for the season finale, right?!