1942. Sadie Gault is eighteen and living with her parents in the Kraków Ghetto during World War II. When the Nazis liquidate the ghetto, Sadie and her pregnant mother are forced to seek refuge in the perilous tunnels beneath the city. One day Sadie looks up through a grate and sees a girl about her own age buying flowers. 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 the two become close, but as the dangers of the war worsen, their lives are set on a collision course that will test them in the face of overwhelming odds.