The Wars of the Roses have put Edward IV on the throne of England, Lorenzo de' Medici's court shines brilliantly, and Duke Galeazzo Maria Sforza plots in Milan. But this is a changed world, and medieval Europe is dominated by the threat from the Byzantine Empire. Sforza, the Vampire Duke, marshals his forces for his long-planned attack on Florence, and Byzantium is on the march.

A mercenary, the exiled heir to the Byzantine throne, a young woman physician forced to flee Florence, and a Welsh wizard, the nephew of Owain Gly Dwr, seem to have no common goals but together they wage an intrigue-filled campaign against the might of Byzantium, striving to secure the English throne for Richard, Duke of Gloucester, and make him Richard III.