In 1696, young Christopher Ellis is sent to the Tower of London, but not as a prisoner. Though Ellis is notoriously hotheaded and was caught fighting an illegal duel, he arrives at the Tower as assistant to the renowned scientist Sir Isaac Newton. Newton is Warden of the Royal Mint, which resides within the Tower walls, and he has accepted an appointment from the King of England and Parliament to investigate and prosecute counterfeiters whose false coins threaten to bring down the shaky, war-weakened economy. Ellis may lack Newton's scholarly mind, but he is quick with a pistol and proves himself to be an invaluable sidekick and devoted apprentice to Newton as they zealously pursue these criminals.
While Newton and Ellis investigate a counterfeiting ring, they come upon a mysterious coded message on the body of a man killed in the Lion Tower, as well as alchemical symbols that indicate this was more than just a random murder. Despite Newton's formidable intellect, he is unable to decipher the cryptic message or any of the others he and Ellis find as the body count increases within the Tower complex. As they are drawn into a wild pursuit of the counterfeiters that takes them from the madhouse of Bedlam to the squalid confines of Newgate prison and back to the Tower itself, Newton and Ellis discover that the counterfeiting is only a small part of a larger, more dangerous plot, one that reaches to the highest echelons of power and nobility and threatens much more than the collapse of the economy.