The English Franciscan monk and scholar Roger Bacon claimed to have studied and seen many marvels of nature and science and concealed these in a book written in an unbreakable code called the 'Secret of Secrets'. Sir Hugh Corbett, Keeper of the Secret Seal of Edward I of England, has been instructed to organise agents in Paris to steal this Book of Secrets. They do so but pay a bloody, violent price and the French King Philip IV now wishes a meeting between the scholars of England and France to discuss the possibility of breaking the code. Edward I has no choice but to allow the meeting to take place, since it is demanded by the Peace Treaty between England and France.

It takes place at Corfe Castle, which becomes a place of murder, mystery and mayhem. Young women from the castle are being slain whilst, in the nearby forest, the outlaw Horehound is deeply concerned by the horrors he has seen there. The situation becomes more serious when two of the French scholars die in rather sinister circumstances. Corbett and Ranulf-atte-Newgate have to thread this maze of malevolent murder whilst, at the same time, trying to decipher the great secrets of one of England's most outstanding scholars.