Giovanni Altofronto, Duke of Genoa, driven from power by the plots of Pietro Jacomo, disguises himself and lives under the name of Malevole at the usurper's court, assuming the character of a malcontent. His identity is known only to his faithful friend Celso. A crafty courtier, Mendoza, who had assisted in dethroning Altofronto, has adulterous intercourse with Pietro's wife, Aurelia. Malevole exposes the intrigue to Pietro; but meanwhile Aurelia, induced by an old procuress, Maquerelle, to believe that her lover is faithless, discards Mendoza and engages in an intrigue with another courtier, Ferneze. Pietro, sword in hand, seeks Mendoza, who makes passionate protestations of his own innocence, and declares that the guilty person is Ferneze. On that very night Ferneze has an appointment with the Duchess; and it is agreed that Pietro with some of the guard shall break into the Duchess' chamber, while Mendoza waits with his drawn sword at the door. Ferneze is to be allowed to escape from the chamber, only to be received on the sword of Mendoza, who is then to stand over the body and pretend that he is guarding it from assault. Thus Mendoza will not only serve Pietro, but by his seeming generosity towards Ferneze will earn the gratitude of Aurelia, who, should she attempt to take vengeance on her husband, will not fail to make Mendoza acquainted with her plots, which he will incontinently reveal to Pietro. At the hour appointed, Pietro and the guard invade the Duchess' chamber; the flying gallant is stabbed by Mendoza and left for dead (though he afterwards recovers from the wound); Aurelia receives Mendoza again into favour, and practises with him to murder Pietro. Mendoza, selecting a time when Pietro had gone a-hunting, bribes Malevole to commit the murder. Malevole undertakes to kill Pietro by stealth in the forest, fling his body into the sea, and then return to announce that Pietro, distracted by grief at the dishonour brought on him by his wife, has made away with himself by leaping into the sea from a high rock. To the forest goes Malevole, finds Pietro, and exposes to him the plot; presently Celso appears bringing a hermit's weeds, into which Pietro shifts. They return to the court, and the pretended hermit describes with much detail how he saw Pietro perish, the narrative being substantiated by Malevole. Mendoza is proclaimed Duke, and his first act is to pronounce a sentence of perpetual banishment on Aurelia. He then commissions Malevole to bring from the citadel (where she is confined) the wife of the banished Altofronto, the virtuous Maria, whom he intends to make his Duchess. His brain is now exercised to procure the destruction of the supposed murderers. Malevole is instructed to poison the hermit at a supper given in the citadel, and the hermit on the same occasion is to poison Malevole; thus two awkward agents will be removed, and the suspicion will fall on Maria, whose fears will drive her to submit to Mendoza.