Twelve-year-old Kat Bateson likes to have a logical explanation for everything. But even she can't make sense of the strange goings-on at Rookskill Castle, the drafty old Scottish castle- turned-school where she and her siblings have been sent to escape the London Blitz. What's making those terrifying sounds at night? Why do the castle's walls seem to have a mind of their own? And why do people keep mysteriously appearing and disappearing? Kat thinks she knows the answer: Lady Eleanor, who rules Rookskill Castle, is harboring a Nazi spy. But when her classmates begin to vanish, one by one, Kat must face the truth about : what the castle actually harbors and what Lady Eleanor is - before it's too late.