London, 1590. Queen Elizabeth I's control over her kingdom is wavering. Amidst a tumultuous backdrop of Spanish plotters, Catholic heretics and foreign wars threatening the country's fragile stability, the body of a small boy is found in the City of London, with strange marks that no one can explain.

When idealistic physician Nicholas Shelby finds another body displaying the same marks only days later, he becomes convinced that a killer is at work, preying on the weak and destitute of London.

Determined to find out who is behind these terrible murders, Nicholas is joined in his investigations by Bianca, a spirited tavern keeper. But when their inquiries lead them to the fearsome attentions of the powerful Robert Cecil, Nicholas is forced into playing to Cecil's agenda, and becoming a spy...

As more bodies are discovered, the pair find themselves caught in the middle of a sinister plot. With the killer still at large, and Bianca in terrible danger, Nicholas's choice seems impossible - to save Bianca, or save himself...