1873. At an abandoned château on the outskirts of Paris, a dark séance is about to take place, led by acclaimed spiritualist Vaudeline D’Allaire. Known worldwide for her talent in conjuring the spirits of murder victims to ascertain the identities of the people who killed them, she is highly sought after by widows and investigators alike. Lenna Wickes has come to Paris to find answers about her sister’s death, but to do so, she must embrace the unknown and overcome her own logic-driven bias against the occult. When Vaudeline is beckoned to England to solve a high-profile murder, Lenna accompanies her as an understudy. But soon they begin to suspect that they are not merely out to solve a crime, but perhaps entangled in one themselves.

Hidden in the depths of 18th-century London, a secret apothecary shop caters to an unusual kind of clientele. Women across the city whisper of a mysterious figure named Nella, who sells well-disguised poisons to use against the oppressive men in their lives. But the apothecary’s fate is jeopardized when her newest patron, a precocious 12-year-old, makes a fatal mistake, sparking a string of consequences that echo through the centuries. Meanwhile, in present-day London, aspiring historian Caroline Parcewell spends her 10th wedding anniversary alone, running from her own demons. When she stumbles upon a clue to the unsolved apothecary murders that haunted London 200 years ago, her life collides with the apothecary’s in a stunning twist of fate --- and not everyone will survive.