Clara Button is no ordinary librarian. While war ravages the city above her, Clara has risked everything she holds dear to turn the Bethnal Green tube station into the country’s only underground library. Down here, a secret community thrives with thousands of bunk beds, a nursery, a café, and a theater—offering shelter, solace, and escape from the bombs that fall upon their city. Along with her glamorous best friend and assistant Ruby Munroe, Clara ensures the library is the beating heart of life underground. As the war drags on, the women's determination is tested to the limits.

Poland, 1944 When twelve-year-old Martha and her little sister Johanna are snatched while out shopping for food and pushed into an army truck with a swastika on the door, they are heartbroken. Terrified they will never see their mother again, the little girls cling to each other as the truck full of stolen children rumbles through the countryside.

Berlin, 1944. ‘It is a great honour to have you here, Herr Hitler.’ Lili doesn’t stumble over the lie, her hand steady as she pours champagne. Everything must be perfect, no one can suspect a thing… Lili Rodenberg and her husband Marius run the Edel, Berlin’s most glamorous hotel. For its wealthy guests, it is an escape from the destruction outside, with its elegant piano bar and fine amber brandy. But Lili is Jewish, a secret she is terrified will end in tragedy for her and their beloved little girl.

England, 1937: After a devastating childhood at Blakely Hall Orphanage, 15-year-old Sandra is released. She finds work as a housemaid, finally able to put her past behind her. But the start of World War Two throws the country into turmoil, and her brother, Alf, is sent away to fight, leaving her completely alone. Germany, 1939: Eleven-year-old Frieda is about to board a ship bound for England with her brother, Kurt. But at the last moment, as Frieda stands on the deck crammed with frightened children, she spots her brother jumping off, back to land.

1909. Eleven-year-old Olive Radley knows that her stepfather is a threat to the two Choctaw girls boarded in their home. When the older girl disappears, Ollie flees, taking six-year-old Nessa with her. Together they begin a perilous journey to the remote Winding Stair Mountains, the territory of outlaws, treasure hunters, and desperate men. Along the way, they form an unlikely band with other children struggling to get by on their own.