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 awaits 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 patron 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 finds an old apothecary vial near the river Thames, she can’t resist investigating, only to realize she’s found a link to the unsolved “apothecary murders” that haunted London over two centuries ago. As she deepens her search, Caroline’s life collides with Nella’s and Eliza’s in a stunning twist of fate—and not everyone will survive.