It's winter, 1683, and the Great Frost has swept into London. By day, thirteen-year-old Thomasina and her friend Anne peddle sweets on the frozen river, hearing rumors of the magical Frost Fair that awakens there at night. They say if you can find it, Father Winter himself will grant any wish you have. And Thomasina has an impossible wish: the return of her twin brother, whose death left her family fractured. But once they discover Father Winter's icy kingdom, Thomasina and Anne quickly realize the Frost Fair isn't exactly what it seems... and that some wishes never come for free.