Duke Altofronto has been banished from Genoa. A political coup staged by Mendoza with the help of the Florentines brings the weak Pietro Jacomo to power through his marriage to Aurelia, the daughter of a powerful Florentine leader. Altofronto, disguised as Malevole, prepares to bide his time until the state wearies of the new duke. Altofronto’s devoted duchess, Maria, waits faithfully in prison for his return, and Celso acts as his secret informant on matters of state.

Altofronto, as the Malcontent, is described as a likable person of marked intelligence and straightforward honesty. He refuses to flatter as others do. On the negative side, however, he is described as more monster than man, more discontented than Lucifer in his fall, a man living on the vexations of others and at variance with his own soul. It is a mixture that makes him seem unpredictable and serves Altofronto well in plotting against his adversaries. This description of him comes from Pietro, who is strangely attracted to the erratic individual known as the Malcontent. It is Altofronto, disguised as Malevole, who tells Pietro that he is being cuckolded by Mendoza. This condition, Malevole declares, is most unnatural, for a cuckold is a creation of woman and not of God. In this way, Altofronto torments Pietro and inflames him against Mendoza.

Incensed by the thought of a relationship between Mendoza and Aurelia, Pietro confronts the minion with accusations and threats to kill him, but Mendoza placates the duke with disparagement of women and their habits, absolving himself of Pietro’s accusations by telling him that Ferneze is the offender against the duke’s marital rights. To prove his point, he suggests that Pietro break into Aurelia’s room that night; should Ferneze try to escape, Mendoza will kill him. The situation occurs as Mendoza had planned: Ferneze is discovered in Aurelia’s room and is, the minion believes, killed in his attempt to flee.

Later, when Mendoza and Aurelia are alone, they plan Pietro’s murder. Aurelia promises to use her influence to have Mendoza made duke of Genoa. Unbeknownst to them, however, Ferneze has not been killed. Wounded, he attracts the attention of Altofronto, who revives and hides the young courtier.

Knowing that Pietro is hunting, Mendoza hires the Malcontent to pursue and murder the duke. Taken in by Altofronto’s apparent willingness to aid him in his villainy, Mendoza outlines the remaining steps to his ultimate goal. With Pietro removed and his alliance with Aurelia established, he will be ready to make his bid for power. The banishment of Aurelia will be an easy step because he will publicize her infidelity to the Florentines. Then he intends to marry Maria, Altofronto’s imprisoned wife, whose friends will strengthen Mendoza and his faction.

Reassured by Mendoza’s admission that he does not love Maria, that she too is only a pawn to him, Altofronto takes heart in his assurance that Maria is still true to him, as Celso has reported. Altofronto suggests to Mendoza that they hire a wretch or holy man to report that he has seen Pietro, bereft of reason because of his wife’s infidelity, throw himself into the sea. He also offers to act as Mendoza’s emissary in winning Maria’s favor.

Instead of murdering Pietro, Altofronto divulges to him the plot against his life and provides him with the disguise of the hermit who is to report his suicide. Pietro, in disguise, gives a vivid description of his own anguished demise while lamenting Aurelia’s unfaithfulness. Mendoza immediately banishes Aurelia. He then instructs Altofronto to negotiate with Maria.

Duped by the supposed hermit, Mendoza sends him after Altofronto, with orders to poison the Malcontent at supper. When Altofronto returns for a letter that will admit him to Maria’s quarters in the citadel, he receives Mendoza’s instructions to poison the hermit.

Altofronto and Pietro encounter the banished Aurelia in abject grief because of her indiscretions and her love for Pietro. Altofronto eases Pietro’s hurt by reminding him that many great men have had unfaithful wives, among them Agamemnon, King Arthur, and Hercules. Maria’s faithfulness to Altofronto is proved beyond doubt when Maquerelle and the disguised Altofronto wait on her to deliver Mendoza’s offer of marriage. In answer to their proposal and the promise of great riches if she will accept Mendoza, she announces that she already has a husband. Banished or in power, present or absent, Altofronto remains her true lord.

Mendoza’s only remaining threat to power is Altofronto, who in the disguise of the Malcontent knows too much of the usurper’s malice. To be rid of him, Mendoza plans to use the fumes of one of the two boxes that had been given him by his intended victim. According to the giver, the fumes of one box will put the person who breathes them to sleep for twelve hours; the fumes of the other box will kill him immediately. Unhesitatingly, Mendoza opens what he supposes is the lethal box under Altofronto’s nose. The box is, in fact, empty, but Altofronto feigns death. Later, he appears at a masked ball given by Mendoza to celebrate the deaths he had planned. In the meantime, spurned by Maria, Mendoza accuses her of murdering the hermit—the disguised Pietro—whom Altofronto has reported dead. The faithful wife welcomes death as a fate better than that of being married to the usurper.

At the ball, Altofronto chooses Maria as his partner. Revealing his identity, he asks her to remain composed so that others will not recognize him. Pietro dances with Aurelia, who repents of her past deeds and vows her undying devotion to him. At a prearranged signal, Mendoza’s three supposed victims—Altofronto, Pietro, and Ferneze—reveal themselves, to the consternation of Mendoza and the joy of the assemblage. Altofronto is immediately restored to his rightful place as duke of Genoa.

Mendoza pleads for his life, but he is summarily ejected from the court. Aurelia and Pietro are given the blessing of the court. Maquerelle is allowed to carry on her pandering in the suburbs. Bilioso, a sycophant who chose to stand with the wrong rather than fall with the right, is summarily dismissed from any further court favor. Altofronto and Maria are joyously reunited.