McGarrett tells Danny a story about two officers responding to a noise complaint at a haunted house in 1940. Marjorie Webb, a retired medium, is found murdered in that same house, after calling about an intruder. Max said she was literally scared to death. Jerry, dressed as a decapitated man, is sitting for Charlie and Grace, who is grounded. Adam and Kono are headed on a weekenn out-of-town trip, when a woman staggers out into the road in front of them. She leads them to a crashed car, but the occupant has disappeared. Kono and Adam find the missing woman, but they're attacked by two men in masks. They get the upper hand, then the first woman knocks out Kono. Danny calls the team to come to the murder scene, spooks them, then he and Eric show how the house was tricked out to haunt Marjorie.  Kono and Eric are driven to a cult compound filled with masked people, and are hung up in a large structure. Grace refuses to participate in Halloween because she's missing a party. Adam and Kono overpower two of the cult members, and escape. Ian Miller, the man who was haunting Marjorie, was later murdered. They discover his co-conspirator was Marjorie's assistant Nikki, who is actually Julie Hillman, a woman who had run away and who Marjorie said was dead. She blamed Marjorie for her father's resulting suicide, and meant to psychologically torture her, but things went too far. Adam and Kono find a nearby house to call for help, only to find out the owner is involved with the cult. Kono quickly disarms him and they call Five-O. They hold off cult members with improvised and borrowed weapons until help arrives. Grace changes her mind and joins Charlie and Jerry for trick-or-treating.