In 1929, life is tough for ten-year-old Mattie Price, born and raised in the back streets of Sheffield. With a petty thief for a father, and a mother who's turned to the bottle, it is left to Mattie and her brother Joe to care for their three younger siblings. Eleven-year-old Victoria Hamilton has all the material possessions that a young girl could want. But her mother Grace is cold and distant, making no secret of the fact that she never wanted a child, and Victoria is sent to boarding school. In 1940, Mattie and Victoria meet at Bletchley Park, where an unlikely friendship between them is born. They can tell no one, not even their families, about their work. But keeping secrets is second nature for both of them...