Vindice tells Hippolito he will be the pandar that Lussurioso asks for. It is revealed that the Duchess' youngest son has raped Lord Antonio's wife and Castiza and Gratiana are shocked. They discuss their father's death.
The Duchess is angry at the Duke for not freeing her son and so decides to physically act upon her previous affections for Spurio. Once he is convinced they are both revenging him through adultery, making him a cuckold. It is revealed that Spurio hates the Duchess. Duke's past adulterous sins (aka the conception of Spurio) revealed.
Vindice is appalled but decides to test his mother and sister's virtues by doing the job.
Spurio hears that Lussurioso will commit adultery and says he will be able to disinherit him and make him bleed soon. Piato reveals Spurio and the Duchess's affair to Lussurioso. He angrily begins towards her chamber with his sword.
Vindice and Hippolito leave- they cannot believe their luck. Spurio enters and is angry that he missed the opportunity to grab Lussurioso before he left to Castiza's but then is happy to discover the events that have taken place. Ambitioso and Supervacuo plead for his release in a way that is clear they don't mean it. The Duke tricks them sending them to say kill him BUT then the Lords enter and plead for his release and along with his plan the Duke sends them to do this. The Duke says he ought to forgive this because he himself has so many adulterous sins in his past.
They are both selfish and although outwardly working together, also working for their own self-interest.
He remains stubborn that the rape he committed was good, even in his last few lines.
They see the Duke tell his servants to say he is away from court on private business. Piato sends the Duke in with the skull, he is poisoned by kissing it and then the brothers reveal who they are. They force him to watch Spurio and the Duchess, who also insult him, and then they kill him. This means they now have their revenge for both their father and Gloriana.
The guard brings them the head and they are ecstatic but pretend that they are very sad. Then Lussurioso comes in and thanks them for freeing him. They discover that they have killed their brother and are distraught. The scene ends with Ambitioso saying that they will get revenge by killing Lussurioso.
He then asks Hippolito to bring his brother Vindice to court, whom he tells the audience will kill Piato. The Lords enter and Lussurioso and the Lords discuss the absence of the Duke and assume wherever he is it is all fine, he is the Duke after all.
In the end, Vindice and Hippolito discuss their new plan to dress the Duke's corpse in Piato's clothes and say that Piato killed him and then fled.
She tries to blame that he would be the only one who could convince her but eventually she repents. It rains and Hippolito and Vindice forgive her, saying she is free from that sin now. The brothers leave. Castiza enters and pretends to be promiscuous so as to punish her mother who says for her not to be and then they reconcile their differences and are happy again.
Lussurioso enters and makes Vindice stab "Piato", then realises it is the Duke and falls for Vindice and Hippolito's plan. The nobles enter and one who tells the truth about the Duke and Piato being seen together is sentenced to death. The Lords say that Lussurioso is to become Duke and a feast is arranged. Vindice and Hippolito plan to seize the opportunity of the feast. Spurio is pleased is father is dead. Lussurioso says to audience that he will banish the Duchess.
Ambitioso and Supervacuo plan to kill Spurio and then Ambitioso plans to kill Supervacuo.
Vindice's band of revengers enter and stab the four at the table (including Lussurioso). They exit. Ambitioso, Supervacuo, Spurio enter and find everyone dead. Supervacuo says he will be Duke and Ambitioso stabs him. Spurio stabs Ambitioso and a noble stabs Spurio.
Vindice runs in with Lord Antonio and pretends he is in shock. The noble who is still alive is sentenced to death for murdering everyone. Lussurioso revives and Vindice reveals everything he did to him. Lussurioso dies. Lord Antonio is made Duke and Vindice reveals to him that he and Hippolito killed the Duke.
They are sentenced to death. Hippolito is upset but Vindice is happy that all of his revenges went to plan. Antonio ends the play by saying that hopefully all of this death will cancel out the treason that has taken place and they can start a-fresh.