After aspiring historian Caroline Parcewell finds out about her husband's infidelity, she takes a walk along the River Thames and discovers an old apothecary vial. Needing a research project for a distraction, she investigates the vial and finds a mysterious link between her vial and an unsolved 200-year-old case known as the "apothecary murders" and the mysterious woman known as Nella, who was said to have sold well-disguised poisons to downtrodden women who needed a way to get rid of the men in their lives.