Eleven-year-old Fairday Morrow is less than thrilled that her family is moving thousands of miles from civilization to the quiet country town of Ashpot, where she's absolutely certain she'll die of boredom. As if leaving Manhattan and her best friend Lizzie, the only other member of the elite Detective Mystery Squad (DMS), wasn't bad enough, Fairday is stuck living in the infamous Begonia House, a creepy old Victorian "fixer-upper" with ugly drapes, a mysterious past, and a creaky, vine-covered iron gate that reads "fear not the unexpected".

Before she can even unpack her DMS pack, strange music coming from behind a padlocked door leads Fairday up a spiral staircase and into a secret room, where an ancient mirror, a brass key, and a strange picture of a red-haired lady are the first in a series of clues that takes the members of Detective Mystery Squad on an amazing adventure and reveals secrets about Begonia House that they never imagined.

When an old newspaper article alerts Fairday to the disappearance of Ruby Begonia, who vanished from Fairday's own attic many, many years ago on the day she was supposed to be married, Fairday knows she can't solve the puzzle alone. With the help of Lizzie—and a few new friends—Fairday begins an investigation into the mystery of the missing Miss Begonia and the unusual, dangerous things currently happening in the house. Can the DMS squad piece together the puzzle before it's too late? Or will whatever's causing trouble in the house find Fairday and her friends first?