A data-obsessed Crimean War nurse returns to Victorian England broken by illness and haunted by the horrors she witnessed. Having overcome countless societal hurdles to professionalize nursing, she now turns her attention to exposing an ugly truth: the lion’s share of death in the war was caused, not by battle, but by the medical and sanitary malpractice of the British Army. Untangling the mortality data, she creates the rose diagram, an arrestingly visual chart that changes the course of military, medical, data science, and feminist history. This is the untold story of Florence Nightingale and her groundbreaking, statistical achievement.