The Duke enters Vienna and is greeted by Escalus and Angelo. Vincentio praises Angelo for a job well done when, prompted by Friar Peter, Isabel petitions the Duke for justice. Angelo informs the Duke that Isabel may be out of her mind on account of her failed attempt to save her brother’s life from the law. Cursing Angelo, Isabel states her case and accuses Angelo of being a lecher, a hypocrite, and a murderer. She avows that Friar Lodowick will vouch for her. The Duke denounces Isabel as a slanderer and has her taken into custody. Isabel's supposed slander is somewhat substantiated when Mariana steps forth and contradicts Isabel's claim to the extent that it was she--Mariana--with whom Angelo had fornicated, thinking Mariana was Isabel. Mariana identifies herself, claiming that once upon a time Angelo, who was her husband-to-be, had wrongly abandoned her. Though acknowledging their past engagement, Angelo denies he had wrongly abandoned Mariana, citing Mariana's infidelity as the cause for breaking off their engagement. While Friar Lodowick is sent for, the Duke tells Escalus and Angelo that they should get to the bottoms of this, that Angelo himself should mete out whatever justice he deems fit, and that he--Vincentio--will absent himself from the proceedings.
As Lucio attests that Friar Lodowick is a scurvy rascal who disparaged the Duke most villainously, Escalus sends for Isabel so that he might interrogate her himself. By and by, almost simultaneously, they arrive, Isabel, escorted by officers, and Friar Lodowick, escorted by the Provost. Lucio identitfies Friar Lodowick as the rascal. Escalus questions Friar Lodowick if it was indeed he who had contrived to have Isabel and Mariana slander Angelo. When Friar Lodowick vows to testify only in the Duke's presence, Escalus condemns him as a villain, promising him punishment as befitting a villain. Friar Lodowick objects, arguing that he is a foreigner and therefore that Vienna's laws do not apply to him, that as a neutral observer he has concluded that Vienna is a crime ridden city where the laws have been rendered impotent due to the laxity of their enforcement. Escalus accuses the friar of slandering the state. Lucio accuses the friar of defaming the Duke. Friar Lodowick denies that he ever has defamed the Duke, arguing that Lucio was the one to defame the Duke if anyone has. Angelo rebukes the friar. Escalus orders his officers to take Friar Lodowick into custody. Friar Lodowick resists arrest, and in the scuffle Lucio removes the friar's hood, revealing Friar Lodowick to be Vincentio, the Duke of Vienna.
The Duke's revelation prompts Angelo to confess his guilt and to beg for a quick execution. First things are first however. He is told to go with Friar Peter who will marry him directly to Mariana. Vincentio then pardons Isabel, telling her, however, that her brother's death couldn't be avoided. Anon, when Friar Peter, Angelo, and Mariana return, Vincentio condemns Angelo to death for committing that which he--Angelo--put Claudio to death. Mariana begs for Angelo's life, arguing that it would be too cruel to make her a widow on the heels of making her a newly-wed bride. Alas, the Duke stands by his decision. Mariana begs Isabel to advocate her cause. Isabel obliges Mariana, but the Duke remains firm in his determination. Presently, to address an additional gross miscarriage of the law, the Duke orders the Provost to bring Barnardine. Escalus expresses his grave disappointment with Angelo during the interim. Anon, the Provost returns. With him are Barnardine, Juliet, and a man who is covered and muffled. The Duke pardons Barnardine, wishing him the best in living his life anew by becoming a reformed man. Then the Duke has the muffled man unmuffled, revealing him to be Claudio. Noticing a sense of relief in Angelo, the Duke tells him that his death sentence is indeed commuted, but Angelo must prove himself to be a reformed man by being a faithful husband to Mariana. Finally, the Duke addresses Isabel, this after condemning Lucio to prison until he marries the woman whom he has taken advantage of. If Isabel will consent, the Duke will take her as his wife, making Claudio his brother.