Mallory and Claudia are helping Stacey unpack from her move back to Stoneybrook when they find an antique trunk in the McGills' attic. Stacey doesn't want the dusty thing, so she gives the trunk to Mallory... who finds an old diary in the bottom of it.

The diary once belonged to Sophie, a girl who lived in Stacey's house in the 1890s. As Mallory reads deep into Sophie's diary, she discovers a mystery. There may be ghosts that haunt Stacey's house!

The Baby-sitters have one big mystery on their hands to solve. Because Stacey's house isn't big enough for her, her mom... and a family of ghosts!