She (plus her ghost best friend, Jacob, of course) are in Paris, where Cass's parents are filming their TV show about the world's most haunted cities. Sure, it's fun eating croissants and seeing the Eiffel Tower, but there's true ghostly danger lurking beneath Paris, in the creepy underground Catacombs.

When Cass accidentally awakens a frighteningly strong spirit, she must rely on her still-growing skills as a ghosthunter -- and turn to friends both old and new to help her unravel a mystery. But time is running out, and the spirit is only growing stronger.

And if Cass fails, the force she's unleashed could haunt the city forever.