1) Giovanni develops an incestuous passion for his sister Annabella that he tries to repent from unsuccessfully. They consummate their relationship. 2) Meanwhile, Richardetto plots revenge against Soranzo for abandoning Hippolita. Grimaldi accidentally kills Bergetto instead of Soranzo in the plot. 3) Annabella, pregnant with Giovanni's child, hastily marries Soranzo. At the wedding, Hippolita tries to poison Soranzo but is herself poisoned by Vasques.

1. Giovanni, recently returned from University study in Bologna, has developed an incestuous passion for his sister Annabella, despite their blood relationship, and the play opens with him discussing this ethical dilemma with Friar Bonaventura. Bonaventura tries to convince Giovanni that his desires are evil despite Giovanni's passionate reasoning, and eventually persuades him to try to rid himself of his feelings through repentance. Annabella, meanwhile, is being approached by a number of suitors, including Bergetto, Grimaldi and Soranzo. She is not interested in any of them, however, and when Giovanni finally tells her how he feels (obviously having failed in his attempts to repent), she requites his love immediately. Annabella's tutoress Putana encourages the relationship. The siblings consummate their relationship. Hippolita verbally attacks Soranzo, furious with him for letting her send her husband (Richardetto) on a dangerous journey she believed would result in his death so that they could be together, then declining his vows and abandoning her. Soranzo leaves and Vasques promises to help Hippolita get revenge on Soranzo, and the pair agree to marry after they murder him. Ironically, Richardetto is not dead but also in Parma with niece Philotis, and is also desperate for revenge against Soranzo. He convinces Grimaldi that in order to win Annabella, he should stab Soranzo (his main competition) with a poisoned sword. Unfortunately, Bergetto and Philotis, now betrothed, are planning to marry secretly in the place Richardetto orders Grimaldi to wait, and Grimaldi accidentally stabs and kills Bergetto instead, leaving Philotis, Poggio and Donado distraught. Annabella resigns herself to marrying Soranzo, knowing she has to choose someone and it can't be her brother. She subsequently falls ill and it is revealed that she is pregnant. Friar Bonaventura then convinces her to marry Soranzo in haste in order to avoid shame and damnation. Meanwhile Donado and Florio go to the Cardinal's house, where Grimaldi has been in hiding, to beg for justice. The Cardinal refuses due to Grimaldi's high status and instead sends him back to Rome. Florio tells Donado to wait for God to bring them justice. Annabella and Soranzo are married soon after, and their ceremony includes masque dancers, one of whom reveals herself to be Hippolita. She claims to be willing to drink a toast with Soranzo, and the two raise their glasses and drink, on which note she explains that her plan was to poison his wine. Vasques comes forward and reveals that he was always loyal to his master, and in fact he poisoned Hippolita. She dies spouting insults and damning prophecies to the newlyweds. Seeing the effects of anger and revenge, Richardetto abandons his plans and sends Philotis off to a convent to save her soul. When Soranzo discovers Annabella's pregnancy, the two argue until Annabella realises that Soranzo truly did love her, and finds herself consumed with guilt. She is confined to her room by her husband, who plots with Vasques to avenge him against his cheating wife and her unknown lover. On Soranzo's exit, Putana comes onto the stage and Vasques pretends to befriend her in order to gain the name of Annabella's baby's father. Once Putana reveals that it's Giovanni, Vasques has the Banditti tie Putana up and remove her eyes as punishment for the terrible acts she has willingly overseen. In her room, Annabella writes a letter to her brother in her own blood, warning him that Soranzo knows and will soon wreak his revenge. The Friar delivers the letter, but Giovanni is too arrogant to believe he can be harmed and ignores advice to decline the invitation to Soranzo's birthday feast. The Friar subsequently flees from Parma to avoid further involvement in Giovanni's downfall. On the day of the feast, Giovanni visits Annabella in her room, and after sleeping with her, stabs her during a kiss. He then enters the feast –at which all remaining characters are present– wielding a dagger on which his sisters heart is skewered, and tells everyone of the incestuous affair. The news immediately kills Florio with shock. Soranzo begins to attack Giovanni, but Giovanni manages to stab and kill him. Vasques intervenes, wounding him then ordering the Banditti to finish the job. Following the massacre, the Cardinal orders Putana to be burnt at the stake, Vasques to be banished and the church to seize all the wealth and property belonging to the dead. Richardetto finally reveals his true identity and the play ends with the Cardinal saying of Annabella
who could not say, 'Tis pity she's a whore?