The episode begins with Castle asking his daughter to join him in their regular game. But Alexis is not in a mood to, as she has bigger decisions to make- choosing her college. She wants to go to a college far from home. She tells her dad that it is time that they holster the guns and play adults. Martha tells castle that Alexis is growing up and that she would like to do her own things. Just then Beckett calls; but he ignores her call. He tells his mom that Beckett and he haven’t been hitting it off well off late; and Martha tells him that it is his fault. Partnering with a rugged cop and showing up at crime scene with a stewardess could definitely be good reasons for this to happen.
Martha tells him that he is punishing Beckett and that he should at least let her know why. He tells his mom that he is going to a crime scene; his last case with Beckett. The victim is David Lock; a currency trader. His boss found in around seven in the morning, with bite marks all over his arm. The COD appears to be blunt force trauma to the head when he fell to the ground. The forensics doctor tells Beckett that they aren’t ordinary bite marks- the killer latched on to the victim’s flesh and tried to tear off the flesh like a pit-bull. They find a lace cuff on the victim’s body; and Castle deduces that it is from the 1870s. They talk to David’s boss, Mike Mathews. He tells them that last week, someone keyed David’s car. David thought it was Charlie, one of the other traders, who was vying for the same promotion. David was killed around 4 am and Charlie shows up at work around the same time. They decide to talk to Charlie.
They break into Charlie’s apartment and find him cuffed to his windowsill. He tells them that it is not safe for them to be here and that he wants them to shoot him. He tells them that he did go to work this morning and that he too was attacked- it was a zombie! He shows them the bite marks on his arm. Next, castle and Beckett are listening into the 911 call Charlie made. He made the call moments after David was killed. Beckett thinks that it was a psychotic break that Charlie went through due to his guilt for killing David; and he made a fantasy story that zombies killed David. Just then Esposito walks in and announces that the bite marks on both the guys’ arms are identical and they don’t belong to Charlie; it belongs to an unknown third party. Just then Ryan bursts into the room. He shows them the footage from the security camera. The guy on the footage was wearing a suit from the 1800s and he had decaying skin.
Castle declares that the killer is a zombie! Next, Castle tells Martha about his new case; he is very excited. Just then Alexis appears and laser tags her dad. He fell for her act last night! Meanwhile, Charlie says that he wont leave the prison as he could turn anytime, and that he is scared that the zombie is not yet caught. Esposito tells the team that there is a gap in the victim’s timeline. The security guard saw David leave home at 2 am and he was killed at 4 am. The team finds out that David was dating a girl named Greta, who was engaged. But when asked, Greta tells them that she had broken up with David a week ago, as her fiancé Tom, had got the whiff of their affair. He had seen some emails that David sent her. She tells them that she doesn’t know where Tom was the whole night.
They figure out that Tom knew where David worked. They bring Tom in. tom tells Beckett that he didn’t touch David. He tells Beckett that he was at the library and that she could check it with the people present there at that hour. Tom’s alibi checks out. Ryan then informs the duo that there was a chemical that was found on David’s shoes and that it appears that he had visited a chemical factory. They track down the address and Beckett and Castle arrive at the factory. The factory has been closed down for a year. They hear footsteps. They find two people running out of the factory. Moments later, the duo is surrounded by a horde of zombies. Beckett announces herself as NYPD; and one of the zombies tell them that they are just zombie walking!! It’s a game. Beckett asks all of them to come with her to the station. But they don’t find a zombie that matches the one from the footage.
One of the zombies identifies our killer zombie- Kyle Jennings. The team goes to Kyle’s apartment. But they find him dead. The doctor affirms that Kyle is David’s killer as the bite marks match his teeth and the blood on Kyle’s clothes matches David’s. Now, who killed Kyle? They bring Kyle into the morgue. Castle wants to know if there is any way of determining whether Kyle was an actual zombie. The coroner is irritated and he tells Castle that Kyle was an alive man, who is now dead. Just then Kyle springs up and walks out of the morgue. Ryan tells the team that he was sure that Kyle was dead. Castle continues with his zombie theories. Ryan wants to know whether castle really believed in zombies; but Castle says a “no”. He only believes in driving Beckett crazy. Kyle is in the ICU and is awake now. Beckett and Castle go to meet him. But Kyle doesn’t remember anything. He says he would never hurt anyone.
Beckett and Castle aren’t sure whether he is telling the truth. Just then the coroner arrives and tells them that there are traces of a particular drug found in Kyle’s blood. It is used to treat motion sickness; but in large doses, it affects the CNS and makes the user extremely compliant. They basically do whatever they are told. So, someone on the zombie walk slipped Kyle the drug and sent him to kill David. Castle knows the guy who could answer their questions- Paul. They had interviewed Paul during their factory raid and they felt that he was under the influence of some drug. They call him in once again. Paul tells them that he does take the zombie drug; but he doesn’t know the name of the guy who supplies it. They show him a picture and he identifies the supplier- Tom. They bring in Tom. Beckett tells Tom that Kyle saw him at the zombie walk.
Moreover, they also know that he is the one who supplies the zombie drug. But there is no way that they can prove that Tom is the one who dosed Kyle into killing David. Beckett knows that Tom is right; they cant prove anything and all evidences point towards Kyle. But Castle thinks that there has to be a way out. Beckett tries talking to Greta; but no luck. They release Tom from custody. Just then Esposito arrives and declares that Kyle is out on bail. Tom is scared and demands police protection. He tells them that they fed Kyle with lies about him and now Kyle could attack him. Esposito escorts him to his apartment. But just as Tom was about to leave; Kyle attacks the car window with a bat. He tells Tom that he made him kill an innocent man and now he is going to kill Tom. Tom admits that he had to get David killed, as David was driving Greta away from him and he had to do something. Tom is sorry that he used Kyle.
Just then Beckett arrives and tells Tom that he is under arrest for David’s murder. It turns out that our zombie is not Kyle; its Castle; and he did a good job! At the office, they thank Kyle for helping them with his costumes. The DA is looking at dropping the charges. Kyle leaves. They feel sorry for him. Beckett reveals that she is attending therapy and that now she is ready to be in a place, where she can accept everything that happened that day. She thinks that its coming down and Castle tells her that he wants to be there when it does. Next, Alexis tries to laser tag Castle once again; but he has his zombie make up on.
Alexis is terrified and Castle earns his 1000 points- he wins. Its over! Alexis is upset; a game that started when she was five is now over and she is going to leave home to graduate. But Castle tells her that things change; and change is good. Her heart tells her that she shouldn’t go far. But in order to do this, Castle has to promise that he wouldn’t drop in on her and giver her, her space. He promises that he will “try”. So, the game isn’t over. The episode ends.