Mary is the daughter of a merchant-seaman and an Irish-born seamstress, and their only surviving child. Her father died when she was 7 or 8. Her mother became too ill to support them by sewing and was forced into prostitution. Mary took to stealing small amounts from her mother's customers. Unable to afford medical treatment, her mother died and Mary was thrown out on the streets. Her father's surname was Lang, but since she was condemned to hang under that name, she uses her mother's surname, Quinn.
Sentenced as a thief at the age of twelve, is rescued from the gallows by a Anne Treleaven posing as a prison warden. In her new home, Miss Scrimshaw's Academy for Girls, Mary acquires a singular education, fine manners, and a surprising opportunity. The school is a cover for the Agency--an elite, top secret corps of female investigators with a reputation for results--and at seventeen, Mary joins their ranks.