When a disheveled young woman walks into King Arthur's court and demands justice, declaring she has been brutally violated by a knight of the Round Table, Arthur vows that the culprit will be tracked down and punished. But young Gildas of Cornwall, now squire to the king's nephew Sir Gareth, has his own problems: His beloved Lady Rosemounde has been betrothed by her father to another knight. When Rosemounde's new fiance turns out to be the knight accused of the vicious crime, Rosemounde begs Gildas for help. He must enlist the aid of his old mentor, Merlin, in order to prove the innocence of the man he feels has stolen the affections of his only love.

Queen Guinevere, trying to head off a rush to judgment, secures a promise from the king to let her be arbiter in the accused knight's case and sends the young man on a quest to discover what women most desire. The Knight's Riddle is a fast-paced mystery in King Arthur's mythic court of Camelot, set (like the original Arthurian romances) in the high Middle Ages, and narrated by the young squire Gildas, whose knightly ambitions revolve solely around his love of the beautiful Rosemounde.