The Excalibur Trick, the first volume in The Wrong Sword, follows the desperate exploits of Henry of Sanbruc - alias Henri of Paris, alias Henri de la Ville Perdue, alias Harry the Rat - as he tries to escape the minions of Geoffrey II, Duke of Brittany, and thwart the duke's plans to conquer the Angevin Empire with the aid of Europe's most famous piece of mythical weaponry.
Readers with a passing knowledge of medieval history may already know that Geoffrey actually existed (although he dies a little later in TXT than he did in history). You might have heard of his big brother, Richard the Lionhearted; or his little brother, John the Somewhat Less Impressive. Those more studied in 12th Century Anglo-Norman politics might even recognize Geoffrey's scary mother, Eleanor of Aquitaine, or his dad, King Henry II of England, originator of the Assize of Clarendon and scourge of criminous clerks. And if you know that Geoffrey existed, you've probably already figured out that Excalibur and Henry (the Rat Henry, not the King Henry) didn't.
If that sort of conflation of the historical and the fantastic disturbs you, and you don't want to read The Wrong Sword, I completely understand. Don't worry about my feelings; as long as you buy the book, you don't have to actually read it. It'll just be our little secret.