Tucked behind a false wall at the end of a crooked London alley, Nella Clavinger keeps shelves of little glass bottles and a ledger full of names. The women who slip into her dim little shop arrive with shaking hands and a story about a husband, a master, or a man who has done them harm. Nella listens, asks no questions, and hides a poison inside something ordinary enough to go unnoticed. Her two rules have kept her safe for years: the poison can never harm another woman, and every name goes into the ledger.

Two centuries later, Caroline Parcewell kneels in the cold mud of the Thames at low tide. She's on the anniversary trip she planned for two and is taking alone. Her fingers close around a tiny glass vial, cool and smooth and very, very old. For two centuries, the river has kept Nella's secrets. Caroline is about to pull them into the light.