Measure for Measure is set in Vienna where vice currently runs rampant and the laws have been allowed to sleep for many years. “The too-kind Duke Vincentio is setting out on mysterious business and leaving the cares of state to his trusted minister Angelo, with instructions to enforce discipline as he sees fit.
Angelo’s first action is to revive the obsolete law condemning seducers to death. He then plans to make an example of the young Claudio and has him arrested for having sex with his betrothed, Juliet. As he is being taken to prison, Claudio begs his friend Lucio to get help from his sister, Isabella, who, upon hearing the news, hastens to her brother.
Meanwhile, Duke Vincentio is going about the city disguised as a friar, watching the way his deputy, Angelo, rules and hoping that he can bring the city, which under his own too-kindly rule has been given over to crime, back to the ways of virtue.
Isabella obtains an audience with Angelo and pleads for her brother’s life. At first her efforts are to no avail, but later, as her beauty rouses a passion in Angelo, he asks for a second interview at which he suggests she might purchase her brother’s life with her sexual favors. Shocked, she denies Angelo and rushes away. In the meantime, Duke Vincentio has visited Claudio in his cell and learned of his true love for Juliet and of their plans to marry as soon as a dowry can be arranged.
Hastening to Claudio, Isabella tells her brother of Angelo’s lustful and illegal offer; but makes it clear to him that, although she would pray for him dead, she will not sin for him living. An angry Claudio is then led away by his jailors. The duke, disguised still as a friar, has been eavesdropping on the conversation and now tells Isabella that she can save her brother without compromising herself if she will follow his plan: Angelo, he confides, has a skeleton in his own closet in the person of Mariana, whom Angelo had long ago contracted to marry but had jilted because her dowry was too small. Inasmuch as their betrothal would have been considered a verbal contract, legally and morally binding in the future, Isabella is persuaded to agree to a tryst at which Mariana, without Angelo’s knowledge, can be substituted in his bed to consummate their old marriage vows.
Isabella and Mariana carry out the plan, and Mariana shares Angelo’s bed, with Angelo thinking he is with Isabella. Angelo, however, still breaks his promise and demands to have Claudio’s head. The friar/duke, however, acts quickly and has the head from another corpse substituted for Claudio’s, convincing both Angelo and Isabella that Claudio is dead.
The next day, the duke returns to the city as himself and meets Angelo and other officials outside the gates, where Isabella accuses Angelo of seduction and murder. The duke leaves and returns again disguised as the friar, acting as a witness for the women and revealing the story and himself. He then sentences Angelo to death, but first orders a real friar to marry him to Mariana. After the ceremony he yields to the pleadings of Mariana and Isabella and spares Angelo’s life. He next has Claudio brought forth and orders him to wed Juliet and make up to her for what she has suffered. The duke himself then sues for Isabella’s hand, saying “what’s mine is yours, what’s yours is mine.”