“Keep calm and carry on.”

That's what Katherine Bateson's father told her, and that's what she's trying to do: when her father goes off to the war, when her mother sends Kat and her brother and sister away from London to escape the incessant bombing, even when the children arrive at Rookskill Castle, an ancient, crumbling manor on the misty Scottish highlands.

But it's hard to keep calm in the strange castle that seems haunted by ghosts or worse. What's making those terrifying screeches and groans at night? Why do the castle's walls seem to have a mind of their own? And why do people seem to mysteriously appear and disappear?

Kat believes 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 uncover the truth about what the castle actually harbors—and who Lady Eleanor really is—before it's too late.