Twelve-year-old Ainsley Galloway and her ten-year-old brother, Tobin, have just moved to the small, picturesque mountain town of Lowry. The town is famously the setting for the blockbuster Legend of Greyhallow trilogy and there are traces of the films wherever they go. Ainsley's parents even bought the film director's former home, which they dream of turning into an inn that caters to fans eager to check out the local Greyhallow Movie Festival.

But when Ainsley and Tobin are fooling around in the attic of their new home, Ainsley unleashes something. Something she doesn't quite understand. And suddenly, purple mist is flooding the town, she's receiving cryptic messages from strangers in medieval garb. Oddest of all, the enigmatic director of the Greyhallow movies, who's been missing for years, has a message for them.

Ainsley and Tobin have opened a passage between the very real magical world of the movies and their own. In a swirl of shifting loyalties, capricious creatures, and movie trivia, Ainsley and Tobin have to figure out what to do--and who to trust--in order to save them all.