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 neighbours, 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 life. Once hired, though, she finds herself trapped in a male-dominated realm of Nazi secrets and power. The lives of these three women are set on a collision course when the unthinkable happens and Kasia is sent to Ravensbrück, the notorious Nazi concentration camp for women. Their stories cross continents – from New York to Paris, Germany and Poland – as Caroline and Kasia strive to bring justice to those whom history has forgotten.