Act IOn his departure from Vienna, Duke Vincentio deputizes Angelo to administer the laws of the city in his place and appoints the wise "old Escalus" as Angelo's assistant. The Duke, who is concerned that he has been too lax in keeping order in the city, in fact has no intention of leaving Vienna. Instead, he plans to disguise himself as Friar Lodowick so that he can monitor the effect that the "precise" Angelo's enforcement of the laws has on the citizens and to see whether the exercise of power causes any change in Angelo. The new deputy's first actions are to shut down all the brothels in the suburbs and to arrest and sentence to death young Claudio for impregnating his fiancée, Juliet. On his way to prison, Claudio is met by his friend Lucio, who promises to ask Claudio's sister, Isabella, to beg Angelo for her brother's life. Lucio seeks out Isabella at the convent, where she has recently become a novice, and tells her of Claudio's plight. Distressed at this news, she agrees to speak to Angelo.

Act IIWhen Escalus tries to persuade Angelo to reduce the severity of Claudio's sentence, the deputy refuses and instead instructs the Provost to have Claudio executed the next morning. Constable Elbow appears before Angelo and Escalus with the "bawd" Pompey and a gentleman named Froth in custody, and Angelo leaves it to Escalus to sort out Elbow's confusing complaint against his two prisoners. Meanwhile, Isabella arrives and pleads with Angelo for her brother's life, all the while being coached by Lucio to be more compelling in her entreaties. Aroused by Isabella's virtue, Angelo instructs her to return tomorrow for his answer. Meanwhile, the disguised Duke Vincentio visits the prison where Claudio and Juliet are being held and speaks with the loving but repentant Juliet. When Isabella meets the next day with Angelo, he tells her that she must have sex with him if she wants to save her brother. Outraged, Isabella declares that she will "tell the world aloud" about Angelo's hypocritical proposition, and he retorts that, thanks to his spotless reputation, no one will believe her accusations. Distressed and alone, Isabella refuses to submit to Angelo's blackmail. "More than our brother is our chastity," she concludes, and goes to prepare her brother for death.

Act IIIDisguised as Friar Lodowick, Duke Vincentio visits Claudio in prison and reconciles him to his death sentence, but when Isabella tells her brother of the deputy's proposition, Claudio loses heart and, much to Isabella's disgust, begs her to submit to Angelo. "O Isabel! …" he exclaims, "Death is a fearful thing." The disguised Duke intervenes. He tells Isabella about Mariana, a lady who was once engaged to be married to Angelo but whom he shamefully rejected after her dowry was lost at sea. The Duke suggests a plan where Mariana, who still loves Angelo, would secretly sleep with Angelo in Isabella's place, thereby reclaiming her fiancé, saving Claudio's life, and preserving Isabella's chastity. Isabella gratefully agrees to this "bed-trick." Meanwhile, Pompey is arrested once more and taken to jail, and so is his employer, the brothel-keeper Mistress Overdone. Unaware that the friar is really the Duke in disguise, Lucio strikes up a conversation with him and—unwisely pretending to know the Duke well—claims that Vincentio is a fool, a drunk, and a libertine. The Duke closes Act III with a pronouncement on his deputy: "Twice treble shame on Angelo, / To weed my vice and let his grow!"

Act IV The disguised Duke, Mariana, and Isabella meet to confirm the details of...