Working in his shop one early afternoon, Apothecary John Rawlings is taken aback when a middle-aged stranger rushes in through the door looking panic-stricken. The man is not seeking a cure for a terrible ailment, however, but a place to hide. John agrees and a moment or two later a formidable woman comes to the counter enquiring if the Apothecary has seen anyone answering the description of the stranger he has in his back room. Honouring his promise to the man, John sends her on her way and, intrigued, goes to question the fellow. It transpires that the man's name is Aidan Fenchurch and he is being pursued by Mrs. Ariadne Bussell, a former lover who is reluctant to give up the chase. So reluctant, in fact, that she has been shadowing him for years. Feeling sympathy for Aidan, John agrees to do him a good turn but is shocked when a few days later the man is found dead. Suspicious, Sir John Fielding sends the Flying Runners to arrest Mrs Bussell, who is taken ill in the coach before she can be thoroughly questioned. The Apothecary recognises the signs of fatal poisoning. As the victims begin to pile up John Rawlings embarks on a journey that leads him to the Surrey countryside in search of answers.