I knew shamefully little about Newton before starting this book (other than the obvious 'scientist who thought up gravity when an apple landed on his head'.) The book describes his early life, and rise to fame as a scientist. (Apples are barely mentioned). Later in life, he was appointed Warden of the Royal Mint, at a time when England's coinage was debased and in need of re-minting. Part of his role as Warden was to chase down anyone who was counterfeiting coins- this was punishable by death, as it was considered treason against the monarch (whose face was on the coinage). Thus, Newton came into conflict with counterfeiter William Chaloner, who first came to London as a runaway apprentice, trying to make his fortune.
(One of my favourite parts of the book was the description of London- it reminded me of Terry Pratchett's description of Ankh Morpork in the Discworld books. I'd always assumed that he'd based that on London, but this is the first description of London that I have read which is as vivid as the description of Ankh Morpork).
Chaloner's character and motivations were less well fleshed out (and thus less interesting) than Newton's- for the simple reason that there is much less evidence of Chaloner's life- Newton seems to have left behind acres of writing.