After their father, a video-game inventor, strikes it rich, the Smithfork kids move from their cozy Brooklyn neighborhood to a swanky apartment on Manhattan's Fifth Avenue.

One day Brid, CJ, and Patrick discover an astonishing secret about their apartment. The original owner, the deceased multimillionaire Mr. Post, long ago turned the apartment itself into a giant puzzle containing a mysterious book and hidden panels—a puzzle that, with some luck, courage, and brainpower, will lead to discovering the Post family fortune. Unraveling the mystery causes them to race through today's New York City—and to uncover some long-hidden secrets of the past.