The Hidden Staircase is the second volume in the Nancy Drew Mystery Stories series written under the pseudonym Carolyn Keene, published in 1930 and revised in 1959. The original text was written by Mildred Wirt Benson, and she has said that it is her personal favorite of the Nancy Drew Books she wrote.

The novel was adapted as a Warner Bros. film, Nancy Drew and the Hidden Staircase, in 1939. Another adaptation of the book from Warner Bros. was released on March 15, 2019.

At the beginning of the original edition of The Hidden Staircase Nancy is home alone while her father and their housekeeper, Hannah Gruen, are both out for the day. The doorbell rings and Nancy is introduced to the "rude visitor," Nathan Gombet, who has come to see Carson Drew about some papers. When his persistence irritates and insults Nancy, she grows impatient and angrily tells him to leave and threatens to call the police.

Soon after, Nancy is surprised at her house by Allie Horner, whom she aided in The Secret of the Old Clock. Allie knows Nathan Gombet by reputation because he tried to steal eggs from her farm. When Carson Drew arrives home, he explains his history with Gombet: Gombet signed over his land to build a railroad and then decided after the construction that he had been cheated out of money.

The following day, Allie introduces Nancy to the Turnbull sisters, Rosemary and Floretta. They ask Nancy to help them learn the cause of the mysterious "hauntings" of their mansion in Cliffwood, several miles from River Heights. The sisters explain that numerous valuable items have gone missing from their Civil War-era mansion, but they cannot understand how any person could have entered the locked home during these different instances to commit the thefts.

After a gap of a few days in which Nancy visits with Helen Corning (a recurring friend in the earliest books in the series) Nancy stays overnight with the Turnbull sisters for a week to determine the source of their problems. She goes alone and has her own bedroom. Before she leaves, her father gives her a revolver from his desk to protect herself. He explains that he will be out of town on business in Chicago for a few days but no more than a week. They decide that when he knows the day of his arrival back in River Heights, Nancy will pick him up at the train station to discuss her findings at the Turnbull mansion.

Before Nancy leaves for Cliffwood, she receives a threatening letter telling her to stay away from the Turnbull mansion. While she is there more items are stolen and at one point they find a couple of canaries flying about the mansion. When Nancy notices a stone mansion across a hill which is almost identical to the Turnbull Mansion, the Turnbull sisters tell her that both mansions were built at the same time by distant relatives of theirs, two brothers who eventually became rivals during the Civil War. Likewise, they explain that none other than Nathan Gombet owns the other stone mansion, and he has even asked to purchase their house.

Armed with this knowledge, Nancy is certain that Nathan Gombet is the culprit behind the thefts and is determined to prove it. Meanwhile, her father, who sent a telegram to Nancy to pick him up on a certain day at the station for their meeting, disembarks to find Nathan Gombet waiting for him. Nathan confuses Carson and leads him to believe that Nancy has had an accident near Gombet’s mansion and is waiting there, in trouble. Distraught and believing Gombet, Carson Drew rides back to Gombet’s mansion, where he is then held prisoner. Nancy never receives the telegram from Carson Drew and worries that something is wrong since she cannot reach him.