As World War II sweeps the continent and England steels itself against German attack, Maggie Hope, former secretary to Prime Minister Winston Churchill, completes her training to become a spy for MI5. Spirited and strong-willed, with one of the sharpest minds in government for mathematics and code-breaking, she expects to be sent abroad to gather intelligence. Instead, she is dispatched to Windsor Castle to tutor young Princess Elizabeth in math. Yet castle life quickly proves more dangerous--and deadly--than Maggie ever expected.