Things aren't as they appear at Government House, as Sam and J.J. soon discover on a field trip to the historic building, when they find themselves discussing mushroom growth with George Watt, the gardener of the property at the turn of the 20th century. Just as quickly as they landed in the past, they're back in the present, mystified as to what happened and how they managed to travel back in time.

When the class is assigned a project for the 125th anniversary of Government House, J.J. and Sam use the opportunity to find more information about the gardener, and have a meeting with Alice, one of the former housekeepers. But before they can begin, they're confronted with a woman that no one else can see, and she attempts to speak to them, silently mouthing the word "watch".

Who is this mysterious woman, and what does her ghost want with Sam and J.J? In order to solve the mystery, the girls must navigate the grounds of Government House – a challenge because Sam and J.J. can't cross them without winding up in the past. And with only the faintest idea of how to return to the present, their forays into the secrets and history of Government House become dangerous. Can they figure out what the ghost wants before they're stuck in the past for good?