Castle and Beckett confront the possibility of paranormal foul play, when a world-renowned ghost hunter is mysteriously murdered while investigating a haunting at a legendary New York mansion

In the opening sequence we see a man,  who seems to be a ghost hunter, searching  in what looks to be an abandon house when  some strange things start to happen. All doors in the house suddenly close, the lights begin to flicker on and off before the power goes out. This ghost hunter starts to say “Mercy, I can see it” over and over before he screams and the scene fades away.

Castle and his daughter Alexis watching a scary movie. Alexis has to go Skype with her boyfriend and Castle tries to tell his daughter that maybe it is not such a good idea of rearranging your schedule based around another person, only to have Beckett call him and drop whatever he is doing to go see the crime scene. Can we say double standard?

At the crime scene Castle notices the Ghost Wranglers van were Castle and Beckett debate the merits of the show. Castle states that its star, Jack Sinclair, is without a doubt the best ghost hunter there is. That is when Beckett reveals that their victim’s name is Jack Sinclair.

Apparently, Jack was at the house tapping an episode for his television show, due to the house’s paranormal history and unfortunately it looks like that history is what got Jack killed. His throat was slashed from ear to ear, but the blood splatter suggest that no one there to do it or there would have been a void in the splatter. Castle suggests that maybe a ghost did it, much to the disbelief of his fellow colleagues. The person who reported Jack’s death is his producer Lulu. Lulu rehashes the day’s events and said they did a thorough sweep of the house and there was no sign of anyone in the house, she however did say that she has video of Jack right before his death. It shows what looks like Jack begging for his killer for mercy, before the feed cuts out.

While all of this is going, Esposito and Kevin are playing a double date so that Jenny and Laine can finally meet. Esposito tells Kevin not to bring up any topics that are controversially which leaves Kevin worrying about double date.

His camera man, who usually helps Jack quit hours before after the two, had an argument. They pick up the camera man, Barry who proceeds to tell him about the argument that he and Jack had. Apparently, Jack didn’t even want to go the house because of some strange aversion of being back in his old neighborhood.  Barry is release from police custody when his alibi checks out. That is when Kevin shows them footage of the video right before Jack’s death showing the camera tripod in the corner moving on its own; leaving the question of what the hell is really going on?!

According to Castle, the house has a long history with murders going back to the 1800s all claiming that a demon was involved. Kevin finds out the reason why Jack didn’t want to visit the house in the first place is because he used to live minutes away from the house when he was a kid. Castle decides to learn more about the house by contacting previous owner much to Beckett’s chagrin, but Kevin steps out to go with him, seemingly becoming the only to believe in Castle’s wild claims ghosts and demons. While talking to the previous owners they say that the living room, same place Jack was killed at, was the epicenter of all the activity. They even went to go see a specialist, a Mercy Legrange. Maybe Jack was pleading for mercy, maybe he was referring to Mercy the specialist?

Beckett and Castle go see Mercy who explains that Jack went to go see her before heading into the house. She says that he was having dreams of repressed memories from witnessing a murder that took place in that house over twenty years ago. The murder that Jack Sinclair witness was that of Melanie Benton whom was slashed to death by a knife. According the detective on that case, a Mr. Smith no one was arrested for the murder and Melanie’s husband Matt was their prime suspect. He supposedly fled to Europe and has never been seen since. Jack however did contact Matt Benton’s brother Pete a week before his death. Pete could have alerted his brother about Jack’s inquiries and could have helped his brother kill Jack.

Castle remembers something about the architecture of the house which leads him to the architect of the house Seamus McClair. Seamus was Scottish grew up near a castle that was known for its alcove or a hidden room where a host can eardrops on its guest above a dining a hall. Seamus was so inspired by this that he put alcove rooms in all of buildings and houses. So there must have been a hidden room in the house that allowed the killer to sneak up on Jack right before he or she killed him. Castle and Beckett go the house where they not only find the hidden alcove, but also the long deceased body of Matt Benton. Looks like he never made it to Europe after all.

Meanwhile, Esposito, Laine, Kevin and Jenny are all enjoying their double date and having a great time, until Jenny decides to ask Esposito and Laine when are they getting married. Not the greatest question to ask when meeting another couple for the very first time.  Esposito and Laine eventually decide to cool things down a bit to figure out where they want their relationship to go from now on.

Castle and Beckett go back to see Pete Benton when they find out that Melanie used be engage to Pete before she left him for this brother Matt. They accuse him of killing the two, but he said he didn’t. Pete says that he always thought that Matt killed Melanie. He always used to have a terrible temper and Pete says that Matt was convinced that Melanie was cheating on him. They come to believe that maybe it was Mr. Fuller the owner of the house so the team decides to setup a sting operation hoping that Fuller will show up

But only to have it be Detective Smith, the detective who they spoke to earlier about Melanie’s death. He was having an affair with Melanie and when she decided to end things he killed her and as for Matt he used to abuse Melanie all the time, so according to Smith he deserved what was coming to him. Beckett arrests Smith for the murders of Melanie, Matt and Jack.  Castle heads home to spend some quality time with Alexis watching flesh eating zombies and still believing in the paranormal no matter what anyone else thinks.