Measure for Measure opens with the Duke of Vienna temporarily leaving his young deputy Angelo in charge of the city. We later learn that the Duke will be watching over him, disguised as a friar. Angelo acts quickly, reviving harshly moralistic laws: the brothel-keeper Mistress Overdone is about to lose her business and Claudio is sent to prison for getting his fiancée Juliet pregnant before marriage. Claudio’s sister Isabella, about to take her vows as a nun, is persuaded by Lucio to plead for her brother’s life. After a comic scene in which the ridiculous constable Elbow accuses Pompey and Froth of corrupting his wife, Isabella begins her pleading. To his surprise, the normally chaste Angelo is aroused both by her powerful arguments and by her chastity. He agrees to spare Angelo if she gives up her virginity to him, warning that if she tells anyone, they will believe his word against hers.
The disguised Duke visits Claudio and Juliet in prison and devises a ‘bed-trick’: Isabella will be substituted by Mariana, a woman Angelo had wooed but abandoned when her dowry was lost. When the Duke learns that Angelo has gone back on his word, he arranges to send him the prisoner Barnardine’s head instead of Claudio’s. But Barnardine claims to be too drunk to be executed, so Ragozine’s head is sent instead. Lucio repeatedly slanders the Duke to the supposed ‘Friar’. The Duke returns to meet Angelo and the old councillor Escalus at the city gates and maintains his pretence of ignorance of what has been going on. When Isabella explains Angelo’s abuse of power, and Mariana explains that they have consummated their marriage, the Duke sides with Angelo, who denies it all. But when the Duke returns dressed as the Friar and is accidentally unmasked by Lucio, all is revealed: Angelo begs to be executed. The Duke condemns Angelo to death after he has formally married Mariana to preserve her honour, but Mariana and Isabella plead for his life. The Duke unmasks Claudio, whom the other characters had thought to be dead, and finally proposes marriage to Isabella.