It's 1584 and antagonism rages between Protestant England and Catholic countries of Europe. Many people, even in England, would like nothing better than to to depose (or kill) Queen Elizabeth and install a Catholic monarch on the English throne. Thus the Queen's adviser, Sir Francis Walshingham, has an extensive network of spies working to sniff out Catholic sympathizers. One of the spies is the Italian ex-monk Giordano Bruno, currently living in the French Embassy in London.
As the story begins, Bruno is hurrying through London when he's surreptitiously approached by Sophia Underhill, an old (sort of) flame he still loves. Sophia has come from Canterbury to seek Bruno's help. Sophia tells Bruno that she was forced to marry Sir Edward Kingsley, an abusive older man who made her life a misery. Sir Edward was recently bludgeoned to death in Canterbury Cathedral and Sophia is accused of the crime. Fearing she'd be hung Sophia escaped to find Bruno, who she thinks can expose the real murderer. She's sure the killer is Sir Edward's son Nicholas - a lout interested only in women and gambling.
Bruno asks his employers' permission to go to Canterbury, which is a suspected haven for Catholic sympathizers. Catholic Archbishop Thomas Becket was murdered in Canterbury Cathedral in 1170, after which he was declared a saint. Many people believe saints' bones can perform miracles, but Sir Thomas's bones have presumably been lost or destroyed.
Bruno goes to Canterbury where he has two tasks: find the real killer of Sir Edward and search for Catholic plots against Queen Elizabeth. Sophia, disguised as a boy, accompanies him. In Canterbury Bruno stays with his elderly friend Dr. Harry Robinson, a Protestant official and spy for Sir Francis Walsingham while Sophia stays with protective Huguenot friends.
As Bruno pursues his investigations he learns that something sinister seems to be going on in Canterbury and that several young boys are missing or murdered. Bruno's inquiries incur hostility from various people, including his host's manservant, the local physician, and the church treasurer. Then, when a local shopkeeper is killed, Bruno himself is accused of murder. This is followed by more murders, and it seems clear that one or more Canterbury residents are covering their tracks about something.
I'm not a history buff and don't read a lot of historical fiction but the depiction of Canterbury's streets, houses, people, and ambiance feels authentic. So does old England's rather hasty (and probably unfair) dispensation of 'justice' at the time. Seems you could be accused of murder (a crime for which you were not allowed a lawyer), tried, and hung all in the space of a week or two!
The book's plot is engaging and the characters are sufficiently well-rounded and believable. There's even a courtroom scene, where Bruno (and others) are tried for their crimes. The book has some twists that surprised me and an almost satisfying ending. Could be some unfinished business is addressed in subsequent books in the series.
Over all I'd say this is a good historical mystery that fans of the genre would enjoy.