December 1703 finds Berry Grigsby living as Mary Lynn Nash in a small Welsh village where she has fallen victim to Professor Fell's involuntary drug experiments. Her mind is quickly deteriorating under the drug's influence, and the only way to save her is a potion book that was stolen in an attack on the village orchestrated by a mysterious madman going by the name Cardinal Black.

Matthew Corbett has volunteered to travel with Julian Devane, a self-proclaimed "fool and bad man" in the employ of Professor Fell, to hunt down the potion book. They follow the trail to London, where the book will be sold at a secret auction. Matthew and Julian manage to secure a seat at the auction by masquerading as respected and feared underground operatives, but to prevail in their high-stakes mis