Today I am going to just give you Wikipedia’s review of Carolyn Keene’s The Hidden Staircase!
In the beginning of “The Hidden Staircase” Nancy gets a call from her friend Helen Corning, telling her of a possible mystery to be solved. Nancy is introduced to Mrs. Flora Turnbull and Mrs. Rosemary Hayes by Helen, who is the great-granddaughter of Miss Flora and great niece of Aunt Rosemary. Miss Flora believes that their home, Twin Elms, is haunted. Nancy has just agreed to help solve the mystery of Twin Elms when a man named Nathan Gomber tells Nancy that her father, Carson Drew, is in great danger. Gomber represents a client and other land owners who are dissatisfied with the proceeds of land purchases for a new railroad. Mr. Drew tells Nancy to go to Twin Elms with Helen and help Miss Flora and Mrs. Hayes, as he will be traveling to Chicago during the first part of Nancy’s stay at the old estate, and will join her later.
After an enjoyable dance with her former prom date, Nancy and her father inspect the railroad construction near the river, but are forced to swim to safety when a runaway truck nearly runs them down. Nancy and Helen pack their bags and head off to Twin Elms, with Helen announcing the news of her engagement to her boyfriend Jim Archer over the weekend, en route. At Twin Elms, Miss Flora explains her theory of a ghost, and explains the disappearance of items both precious and little value, and strange things happening, such as music playing with no explanation. A chandelier moving by itself, and a mask held up in the windows over the next few days lead Nancy to investigate the interior and exterior of the Colonial house thoroughly. Nathan Gomber, coincidentally, is trying to buy the historic home in spite of protests from the Turnbull family. Although an adjoining estate with a twin home is available for purchase, it is run down and not of interest to Gomber.
In the meantime, Nancy’s father fails to return from Chicago, and after tracing him to the train station, she is led to believe from eyewitness accounts that he has possibly been drugged and kidnapped. Nancy continues to search for clues to her father’s whereabouts and also to solve the mystery of the intruder at Twin Elms, helped by a night watchman and discovery of a secret room. She and Helen decide to investigate the neighboring estate for clues to a secret entrance, since the house was built with the same design as Twin Elms. Read more if you want to find out the end, but I am warning you, spoiler alert!