Leonardo pays a debt to Cesare Borgia that involves him in a deadly conspiracy with Cesare's condotierre.Cesare Borgia is enamoured with designs of battle machines he discovered in Leonardo da Vinci's workshop. In "Artists and Assassins", the previous book, Cesare offered his assistance to save Leonardo from assassination and to help with Leonardo's investigation into destroyed Florentine art work. Ultimately Cesare facilitated Leonardo's disappearance...