Rule #1: The poison must never be used to harm another woman. Rule #2: The names of the murderer and her victim must be recorded in the apothecary’s register.   One cold February evening in 1791, at the back of a dark London alley in a hidden apothecary shop, Nella waits for her newest customer. Once a respected healer, Nella now uses her knowledge for a darker purpose—selling well-disguised poisons to desperate women who would kill to be free of the men in their lives. But when her new customer turns out to be a precocious twelve-year-old named Eliza Fanning, an unexpected friendship sets in motion a string of events that jeopardizes Nella’s world and threatens to expose the many women whose names are written in her register.   In present day London, aspiring historian Caroline Parcewell spends her tenth wedding anniversary alone, reeling from the discovery of her husband’s infidelity. When she discovers an old apothecary vial