Andrea Orsini (Tyrone Power), an artist with both brush and sword, serves Cesare Borgia (Orson Welles) in many capacities. Assigned to offer the hand of the newly widowed Lucrezia Borgia to Alfonso d'Este of Ferrara, a region Borgia wants to own or conquer, Orsini first disposes of some paintings. Camilla Verano (Wanda Hendrix), wife of the Count of Citta del Monte, meets him at his agent's home and Orsini gallantly gives her a painting he was about to sell for 100 ducats. Borgia has almost promised Orsini that he will be allowed to marry Angela Borgia (Marina Berti), another sister. Coming home from a gondola ride with Angela, Orsini is attacked and the attempted assassin, Mario Belli (Everett Sloane), joins his mission.
On the way back from his successful mission, Orsini stops at the home of a blacksmith's widow who, we learn, is his mother (Katina Paxinou) and he Andrea Zoppo, a peasant, instead of a relative of the great Orsini family of Italy. One of his paintings hangs on her wall, a Madonna to whom she prays that he be punished for his evil ways. Belli spies on this meeting.
Andrea's next assignment is ambassador to Citta 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 jealous of the countess. With Belli, Orsini spies out the city which is truly perched on a mountain. He learns that the old man loves gardening and roses, and that when he has a problem to solve, he climbs to a precipice easy to fall from that looks out over Italy. Belli is delighted because it will be easy to kill him, but Orsini is impressed by the count's love for his people.
When Borgia offers peace with surrender, the count defies him and Orsini joins his cause, but Belli returns to Borgia. The count dies after the first battle and Orsini takes up the defense. After three months the city is at its last extremity. Borgia offers terms that the countess Camilla rejects in favor of battle. When it ends, Orsini gives himself up to Borgia's troops at the price of putting the former terms in writing.
At a triumphal dinner, Borgia exposes "Orsini" to the countess as a peasant and brings Andrea's mother to confirm the fact. He orders Andrea's death but Belli begs to put out his eyes and leave him a blind beggar. He then fakes the torture, with Andrea's help, and Mother Zoppo takes him home.
When he has recovered Belli visits and the two plan to help the citizens retake the city. When Borgia's chief henchman rings the church bell too early, the plan goes awry and Cesare Borgia winds up dead under Andrea's sword. All's well that ends well and the peasant marries the countess.
Most of the scenes were shot on the exact locations in Italy.