In Rome, in the year 1500 a funeral is taking place in an ornate church. In the first pew, two noble women in mourning are gossiping, and the man next to them orders one of them to show proper respect for her dead husband. The man soon leaves the church and signals others to join him.In a large chamber, the man, Cesare Borgia (Orson Welles) tells others that his goal is to unify central Italy under his control. By military means perhaps, but if a suitable marriage is arranged it will happen more quickly. He wants to marry his just widowed sister Lucrezia to Alfonso d'Este, heir to the duke of Ferrara, and he needs the right ambassador. The old duke's death might happen soon after.The man he needs for the mission must be "as quick at deceit as a fox. He must have the grace of a dancer, the wrist of an assassin. He must have little regard for good faith, yet by his astuteness be able to confuse men's minds . . . He must charm as a snake charms birds, yet he must make no friends, except for those who may be of use to him, and for the same reason although he may make use of love, he must not love."Cesare looks around the room, carefully, slowly, then tells everyone to leave except Andrea Orsini (Tyrone Power), young, smart, charming, deadly with a blade, opportunistic, much as his master. Andrea has been courting, with Cesare's acquiescence, Angela Borgia (Marina Berti), cousin to Cesare.The mission is to start at once, which much displeases Angela, but Andrea soothes her feelings, assuring her that his mission will be over quickly. Since he is himself a skillful painter, his first step is to paint a portrait of Lucrezia to take with him to the proposed groom. He needs money, so he offers another of his canvases for sale to an agent. While haggling with the agent, 25 ducats, not less than 75, maybe 30, etc., a handsome young woman observes him and the painting, and offers 100 ducats. She is Camilla Verano (Wanda Hendrix), wife of the Count of Città del Monte, and Andrea gallantly gives her the painting, refusing payment. The script is studded with gallant, clever, cynical observations about love, diplomacy and war, as in this scene.Shortly before departure, as Andrea walks back to the palace with Angela, a hired assassin first stalks the couple, then moves in to stab Andrea with his dagger. However, Andrea has spotted him, his lunge fails, and Andrea overpowers his attacker, Mario Belli (Everett Sloane). Orsini makes Belli an offer he can't refuse, his life for information and a switch of allegiance. Since an assassin who reveals who hired him would be marked for death anyway, Belli agrees to serve Orsini, solemnly promising to give warning before switching allegiances again. He had been hired by the Duke of Ferrara, who hated Cesare with a passion that extended to any of Cesare's ambassadors or even servants.Mario Belli joins Andrea Orsini in travel back towards Ferrara, riding on a canal barge. They stop at an inn and Andrea sneaks out and heads for the home of a blacksmith's widow. Belli follows, unseen.The blacksmith's widow is Mona Zoppo (Katina Paxinou), and we learn she is Andrea's mother. He is a commoner, an impostor, not a member of an obscure side branch of the family of the powerful Orsini family of Florence, as he publicly claims. Mona is happy to see her son well, yet is horrified when she learns he is an impostor and deceiver. She prays to a painting of the Madonna that Andrea did when he was young, that he might leave his sinful ways and live with honor, even with little money, or if not, that he be punished. The altercation with his mother is witnessed by Belli, who has followed Andrea surreptitiously.Andrea and Mario go on to Ferrara, and seek out Alfonso dEste, the intended husband for Lucrezia, who is a genius at military engineering. With a lot of flattery and deceit and promises of a doubling of the usual dowry, they get Alfonso to sign a marriage contract, despite the old Duke's angry threats.Returning to Rome, Andrea's next assignment is as ambassador to Città del Monte, with orders to find a way to remove the elderly count or to help Borgia conquer the city. Angela is not happy about this because she is wary of the young countess, but once again, Angela's wishes give way to affairs of state. At Città del Monte, Andrea soon ingratiates himself with Camilla and her husband, seventy year-old Count Verano (Felix Aylmer), ruler of the fortified town.With Belli, Orsini cases the defenses of the city, which is truly perched on a mountain. They learn that the old man loves gardening and roses, and that when he has a problem to solve, he goes to meditate at a high terrace overlooking a valley, next to a precipice. Belli is delighted because it will be easy to push him from the terrace into the precipice.Andrea observes, both puzzled and fascinated, how the Count rules with wisdom, humanity, patience and honor, in contrast to the style of Cesare. The Count warns him that he is aware of Andrea's likely treachery, yet treats him as though no malice were suspected. As time passes, Andrea begins to admire the Count, and as he knows Cesare Borgia will soon arrive with an army to demand the capitulation of Città del Monte, he discovers that most uncomfortable of feelings . . . a conscience. The fatherless Orsini is increasingly impressed by the Count's nobility, and love for his people, and he also becomes increasingly enamored with Camilla.Orsini takes up painting as a pastime, in a canvas he uses Belli in a "last supper" scene as a model for Judas. Camilla, at first wary of Orsini, grows to admire his artistic talent, which proves to her he has a noble soul behind the warrior and crafty ambassador role. Camilla asks Andrea to paint her portrait, and the Count agrees. With their increased time together, his feelings deepen, and threaten to turn him against his deadly master, whom no one betrays twice. The young woman appears to reciprocate his feelings, but there is no hint that she won't remain a faithful wife.Meantime, Cesare presses for actions that would put Città del Monte under his control. Borgia demands that Verano allow free passage of troops through the territory, and the contribution of additional troops to his service. Verano says he will have an answer the next day, and goes to his high terrace to reflect and pray. Belli recognizes the moment to act has come, tells Orsini he is on his way to the terrace.Orsini suffers a change of heart when asked to betray the noble count and his beautiful young wife, runs after Belli and aborts the planned assassination. Belli announces that he quits his service to Orsini and returns to Borgia.The aging ruler puts the question to his subjects, who urge him to resist the tyrant. Orsini then renounces his service to the Borgias and pledges himself to the cause of defending Città del Monte against them, and quickly organizes the defense and preparations for a long siege. Count Verano is sincerely grateful for Orsini's technical war knowledge and energy. The night before a battle is expected, Verano reveals to Orsini that he now believes he is brave and of noble heart and would be worthy of Camilla in the right circumstances.As Cesare's army passes through the Verano territories, the Count launches an attack on the invading troops as they cross some misty woods. A rousing battle takes place, the cavalry of the invaders retreats in disorder, not without cost, as Verano is mortally wounded. On his deathbed, Verano publicly says he would approve a marriage between Orsini and Camilla.Cesare's armies return, and there is a full-blown castle siege. Fat, stubby cannons blast away from the crenelles, great wooden trebuchets hurl stones and flaming balls of pitch into the city, vats of boiling oil are tipped over onto Borgia's soldiers as they try to scale the city's walls. The city holds well, but after three months the city is exhausted.Borgia sends ambassadors, offering to spare and protect the city and its inhabitants provided Camilla accepts a Borgia appointed prime minister and provided the traitor Orsini is surrendered. Camilla rejects the terms, saying she prefers battle, and the ambassadors return to Cesare. However, Orsini knows the battle would be lost, and he willingly gives himself up to Borgia's troops demanding only that the terms of the offer be put in writing.At a triumphal dinner, Borgia wants to expose "Orsini" to Camilla as a commoner and brings Andrea's mother in. The bound and tortured Orsini is brought in front of the main dining table, and the mother and son react in a way that clearly reveal the truth.Cesare orders Andrea's execution, but Belli convincingly proposes a much more cruel punishment for the traitor, to put out his eyes and leave him a blind beggar forever. Cesare likes and adopts the idea, and Belli says that for the amusement of those present, he can make the sentence effective on the spot, with his two thumbs.Belli screams and yells in glee when he is given the go ahead, but what he actually does is get Orsini to scream bloody murder while he pretends to gouge his eyes out, leaving him dripping with blood and exhibiting two large blood covered grapes as the gouged out eyes. This is too disgusting even to Cesare, so he orders his mother to lead him out and away.Orsini is recovering at his mother's house when Belli shows up to visit. When asked why he betrayed Cesare and saved Orsini, as there appeared to be no payoff, he answers that he did it for professional pride as a doublecrosser. "I discovered that the devil doesn't always pay best. This whole thing pleases me. Who betrayed who and where did it start? No matter. It shall be said among my fellow practitioners in double-dealing that I was the greatest of them all."They proceed to plan a rescue of Camilla from a dungeon and the expulsion from the city of Cesare's soldiers. Citizens of Città del Monte gladly cooperate in the conspiracy. At the last minute, when Orsini has just killed one dungeon guard, knocked out another, things go awry, a church bell signal is given too early, but no matter, the conspiracy succeeds after a canonical sword fighting duel between Don Esteban (Leslie Bradley), the garrison leader, and Andrea, while Camilla watches on.All's well that ends well and the commoner marries the countess.