In 1929, life for ten-year-old Mattie Price, born and raised in the back streets of Sheffield, is tough. With a petty thief for a father and a mother who turns to the bottle to cope with her husband’s brutish ways, it is left to the young girl and her brother, Joe, to feed and care for their three younger siblings. But Mattie has others rooting for her too. The Spencer family, who live at the top of the same street, and Mattie’s teachers recognise that the girl is clever beyond her years and they, and Joe, are determined that she shall have the opportunity in life she deserves.

Victoria Hamilton, living in the opulence of London’s Kensington, has all the material possessions that a young girl could want. But her mother, Grace, a widow from the Great War, is cold and distant, making no secret of the fact that she never wanted a child. Grace lives her life in the social whirl of upper-class society, leaving Victoria in the care of her governess and the servants. At 11 years old, Victoria is sent to boarding school where, for the first time in her young life, she is able to make friends of her own age.

Mattie and Victoria are both set on a path that will bring them together at Bletchley Park in May 1940. An unlikely friendship between the two young women is born and together they will face the rest of the war keeping the nation’s secrets and helping to win the fight. They can tell no one, not even their families, about their work or even where they are. But keeping secrets is second nature to both of them …