After their father, a video-game inventor, strikes it rich, the Smithfork kids find they hate their new life. They move from their cozy Brooklyn neighborhood to a swanky apartment on Manhattan's Fifth Avenue. They have no friends, a nanny who takes the place of their parents, and a school year looming ahead that promises to be miserable.

And then, 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.