John Rawlings is relaxing at the popular swimming baths, the Peerless Pool, when his peace is shattered by the alarming discovery of a drowned woman. The badly beaten victim is identified as Hannah Rankin, an assistant at the nearby St Luke’s asylum for the insane.

Assigned to the case by the Blind Beak, Rawlings visits St Luke’s where he meets one of the inmates, the beautiful Petronelle. She lives in mortal fear of Hannah, "the wicked one". Hannah's references are Rawlings's only clues; they are signed by two people who, by strange coincidence, both come from Bath.

As he probes further Rawlings comes across two mysterious cases of child disappearances; one from Paris, one from Bath, both many years before. Little does he realise that he's scratching the surface of something far more sinister than an isolated murder. Could the cases possibly be connected? And why does Rawlings find his investigation leading him again and again to Bath?

London's most colourful apothecary finds himself in more danger than ever before as he struggles to discover whose evil hand lies behind the trail of death and despair.