Nancy resolves to help Helen Corning's relatives solve the mystery of the ghost haunting their old mansion. A mysterious man appears at the Drew home to warn Nancy that her father is in danger. This warning prompts a search for the missing Willie Wharton, a land owner, who can prove he signed away his land to the railroad and save the railroad from a lawsuit. Meanwhile, the disappearance of Mr. Drew, thefts and mysterious goings on at Twin Elms, and the discovery of a hidden staircase lead Nancy to solve these baffling mysteries.

Books 1-4 share the same ISBN.