There's paranormal activity happening at the old McClaren Mansion in New York and Jack Sinclair, a television ghost wrangler, is called in to investigate the scene. While he's filming from inside the mansion, screams are heard from inside followed by silence. His body is found in the living room with his throat slit and blood splatter on the floor. The team speaks with Jack's television producer who told them that the signals from the camera were going bad just before the murder occurred. Steve Fuller, the owner of the mansion says that he's had trouble renting the building over the last 20 years because of its history.
They suspect Barry Bevetta, the camera man, may have tampered with the video signal but he has an alibi. Castle's ghost theories which had originally been discredited suddenly become more credible when they see footage of a tripod moving all on its own. This is not the first murder where the killer had claimed that demons were responsible.
Fingerprints are found that belong to Ron Berger, recently released on parole who had been embarrassed once on Jack's show. He had written a threatening letter to the show after it had turned him into the laughing stock of the jail. Jack apologized to Ron and asked him to come take photos of the house. These photos appeared in the living room of the mansion.
Castle and Ryan question Mercy LaGrande, another ghost hunter. Jack had screamed out her name before he died. Mercy tells them that Jack had asked for her help because he had been suppressing memories of a murder that had happened in the mansion long ago when he was a young boy. When Jack returned to the mansion, the scene of the crime, he was able to clearly see the killer's face. When Jack was 11, he witnessed the murder of a woman named Melanie Benton. Detective Addison Smith, who was assigned to the case, had suspected that the killer was Matt Benton – the victim's husband. Matt sent postcards from abroad to his brother Pete blaming the murder on demons.
Castle returns to the mansion at night with Beckett. After their candles are blown out by a draft, they use their proton packs and discover a hidden passageway in the ceiling. They are led to a magnetic field generator and the 20 year old skeleton of Matt Benton.
Pete was convinced that Matt had murdered his own wife but he also believed that the victim was having an affair. Castle and team believe that Steve Fuller, the owner of the mansion is the actual killer, especially after finding his fingerprints in the hidden room with the generator. They set a trap by telling Fuller that they will tear down the wall that leads to the hidden room. If Fuller removes the body, they'll know he's to blame. They catch someone in the act but it's not Steve Fuller – it's Detective Addison Smith. Smith was in love with Melanie and killed her after she tried to end their affair. Smith also asked Fuller to give him a call any time there was activity at the Mansion which is how he knew about Jack Sinclair's arrival. He used the legend of the demons to scare people away and confesses this, unknowingly, to a hidden camera.