In 15th century Italy, Bellarion, an orphan, is raised isolated from the world in a monastery. Having exhausted all the learning available to him, he obtains leave to travel to a distant monastery to study Greek. Along the way, he is robbed of his papers and taken for a thief himself. In his escape, he stumbles into the palace garden and meets Princess Valeria, who harbors him from his pursuers.
From there, the plot gets complicated. Indebted to the princess, he assumes the role of knight-errant in fulfilling her aim of deposing the current regent (Marquis Theodore) and replacing him with her brother, the rightful prince. This he does in a roundabout way over the course of several years of scheming, backstabbing, political intrigue, and graphic violence. Along the way, Bellarion is adopted by Facino Cane, the renowned condottiero, and advances himself through his military prowess until, by the end, he is himself a prince.
For reasons too detailed to do justice in a few lines, Valeria comes to believe that Bellarion is a tool of the regent and so does all she can to work against him — at one point, being complicit in his almost-execution. However, Bellarion never loses sight of his mission and at last sees it through. After deposing the regent, he intends to abandon his titles and return to the monastery. Valeria, at last realizing Bellarion’s faithfulness, asks him to marry her.