NEW YORK TIMES BESTSELLER • For readers of The Nightingale and Sarah's Key, inspired by the life of a real World War II heroine, this powerful debut novel reveals an incredible story of love, redemption, and terrible secrets that were hidden for decades.New York socialite Caroline Ferriday has her hands full with her post at the French consulate and a new love on the horizon. But Caroline's world is forever changed when Hitler's army invades Poland in September 1939—and then sets its sights on France. An ocean away from Caroline, Kasia Kuzmerick, a Polish teenager, senses her carefree youth disappearing as she is drawn deeper into her role as courier for the underground resistance movement. In a tense atmosphere of watchful eyes and suspecting neighbors, one false move can have dire consequences. For the ambitious young German doctor, Herta Oberheuser, an ad for a government medical position seems her ticket out of a desolate...