London, 1591. Nicholas Shelby, physician and reluctant spy, returns to his old haunts on London's lawless Bankside. But, when spymaster Robert Cecil asks him to investigate the dubious practices of a mysterious doctor from Switzerland, Nicholas is soon embroiled in a conspiracy that threatens not just the life of an innocent young patient, but the overthrow of Queen Elizabeth herself. With fellow healer and mistress of the Jackdaw tavern, Bianca Merton, again at his side, Nicholas is drawn into a sinister world of zealots, charlatans and dangerous fanatics.