1942. Sadie Gault is eighteen and living with her parents amid the horrors of the Jewish Ghetto in Krakow, Poland, during World War II. When the Nazis liquidate the ghetto, Sadie and her mother are forced to seek refuge in the sewers beneath the city. Ella Stepanek is an affluent Polish girl living a life of relative ease with her stepmother, who has developed close alliances with the occupying Germans. While on an errand in the market, she catches a glimpse of something moving beneath a grate in the street. Upon closer inspection, she realizes it's a girl hiding. Ella begins to aid Sadie and their lives are set on a collision course that will test them in the face of overwhelming odds.