Count Lodovico is devasted to have been banished from Rome—though he admits that he committed crimes (including murder) to warrant this punishment. Antonelli and Gasparo, Lodovico’s friends, promise Lodovico that they will get the banishment commuted as soon as they can.
Back in Rome, the Duke of Brachiano is desperate to get out of his marriage to Isabella and to get into bed with the beautiful Vittoria, who’s married to Camillo. With the help of Vittoria’s brother Flamineo, Brachiano plots to get rid of Camillo. Vittoria’s maid Zanche and Cornelia, Vittoria and Flamineo’s mother, overhears the conversation. Horrified, she interrupts, scolding her children for bringing shame upon their family. But Flamineo counters that he has no other choice: he was born without money, and the only way to get rich is to do the bidding of a wealthy man like Brachiano, no matter how immoral that bidding might be.
Meanwhile, Isabella returns to Rome after traveling. She discusses her marriage with her brother, the Duke Francisco of Medici, and with a cardinal named Monticelso. Francisco has heard about Brachiano’s romance with Vittoria, and he is angry that Brachiano is treating his sister so carelessly. Francisco and Brachiano meet, and Francisco threatens to go to war to protect Isabella. Before violence breaks out, however, the two men are interrupted by Brachiano’s winning young son Giovanni. Because of Giovanni’s charm, Brachiano and Francisco (temporarily) agree to a truce.
Brachiano tells Isabella that he wants to end their marriage. Ever devoted, Isabella decides to protect Brachiano from Francisco’s wrath by pretending that she is the one who has asked for a divorce.
Meanwhile, Camillo learns that someone has thrown horns through his window, a sure sign that the villagers now view him as a cuckold (a man who has been cheated on by his wife). Monticelso convinces Camillo to go away for a while, with the hopes that time apart will increase Vittoria’s desire for her husband. Once Camillo leaves, though, Monticelso and Francisco reveal their true plan: with Camillo out of the way, Brachiano will act on his lust, and they then will be able to catch him in the act.
That night at midnight, Brachiano meets with a conjurer who helps him plan the murders of both Camillo and Isabella. To kill Isabella, the conjurer will have his assistants poison the picture of Brachiano she always kisses before bed; as soon as Isabella’s lips touch the poisoned picture, she will die. To kill Camillo, Flamineo will arrange to go to a horse-vaulting contest with his brother-in-law—Flamineo will break Camillo’s neck, making it look like an accident that the horse caused. Brachiano approves of these plans. The conjurer also tells Brachiano that Lodovico is secretly in love with Isabella and that he will be determined to avenge her death.
A few days later, both Isabella and Camillo are dead and everyone is panicking about Camillo’s death. Francisco and Monticelso suspect Vittoria is at fault, but they have only circumstantial evidence tying her to the crime. However, Marcello—Vittoria and Flamineo’s other brother—knows that Flamineo is guilty, and he chastises his brother for committing such a heinous crime.
Francisco takes Vittoria to court, where a pretentious lawyer questions her. When Vittoria refuses to answer the lawyer’s convoluted questions, Francisco and Monticelso take over, calling her a “whore.” Monticelso shows the court a scandalous letter Brachiano has written to Vittoria—but Vittoria points out that a letter doesn’t prove she slept with Brachiano. Still, the jury—made up largely of ambassadors from other European countries—sides with Francisco, and Vittoria is sentenced to a house of convertites (a house for “penitent whores”).
Using a secret list of criminals that Monticelso has compiled, Francisco comes up with a plan: he will hire Lodovico, a known murderer, to assassinate Brachiano. Before he does that, however, he hopes to turn Brachiano against Vittoria by writing a fake love letter to her. When Brachiano sees the letter, he is initially furious at Vittoria, but she defends her honor and the two gradually make up. Moreover, Brachiano—gaining inspiration from ideas written in Francisco’s letter—decides to escape with Vittoria to Padua, where they can get married in peace. Flamineo vows to follow them, reflecting that “knaves do grow great by being great men’s apes.”
The Pope dies, throwing all of Rome into confusion. Vittoria and Brachiano seize this moment to quietly make their exit, and Monticelso is named the new Pope. When he learns that Brachiano and Vittoria have escaped, he orders them excommunicated. Later that day, Lodovico tells Monticelso that he and Francisco are plotting to assassinate Brachiano—and though Monticelso pretends to be horrified, Lodovico learns from Francisco that the new Pope is actually helping to fund the plan.
In Padua, the tension between Flamineo and Marcello escalates, and Flamineo kills Marcello. Grief-stricken, Cornelia tries to stab Flamineo—but she cannot bring herself to do it. Instead, she descends into madness, and Flamineo starts to feel real guilt.
Brachiano is visited by a handsome Moor named Mulinassar—who is secretly Francisco in disguise. Lodovico and Gasparo have also come to Padua, dressed as capuchin monks. Lodovico quietly poisons the front part of Brachiano’s helmet (the beaver), and as soon as Brachiano puts on the helmet he collapses. Before Brachiano takes his final breath, Vittoria learns that he has left his entire fortune to her. Young Giovanni immediately takes his father’s place as duke, and Flamineo notes that the young man has already become “villainous” like the other powerful men.
Fearing that he’ll be found out, Flamineo tries to convince Vittoria and Zanche that they should join him in a triple suicide to avoid being tortured or killed. Privately, Zanche and Vittoria conspire to ensure that only Flamineo dies—they will persuade him to shoot himself first, and then they will escape from Padua with Brachiano’s money. Flamineo shoots himself, and Vittoria and Zanche rejoice. But Flamineo reveals that he was merely testing them, and that the gun he used was fake.
In the play’s final moments, Lodovico and Gasparo come to execute Vittoria, Flamineo, and Zanche. Flamineo is overcome with sadness and regret, while Vittoria and Zanche stay bold in the face of death. Giovanni, now a duke himself, surveys the bloody scene and plans harsh punishments for all involved.