Vienna is a society under pressure: war with Hungary; a brothel on every street corner, and the constabulary made up of misfits and undesirables. Duke Vincentio, the governor, believes that the city is in moral decline so he has enacted harsh laws to bring the people to their senses. However, as the play begins, we find him getting ready to leave Vienna and pass the unenviable task of the enforcement of these laws to his deputy, Angelo.

When Claudio makes his fiancée, Juliet, pregnant Angelo decides to bring down the full weight of the law upon him and condemn him to death in order to make an example of him.

Claudio’s sister, Isabella, who is about to enter a convent as a novice nun, hears news of the sentence and hurries to Angelo to beg for mercy on behalf of her brother. Angelo denies her request but as she persists he is overwhelmed by lust for her and tells her he will think about it, and that she should return the next day to hear his verdict. She returns as he asks and he tells her that he will pardon her brother if she sleeps with him.

The Duke, in the meantime, has not left Vienna at all but disguised himself as an itinerant friar and is moving about among the people to observe the effect of Angelo’s rule.

Isabella is horrified by Angelo’s proposition and refuses him. She visits Claudio in prison and tells him about it, making it clear that she will not subject herself to Angelo’s demands, and that he, Claudio, will have to die. The Duke overhears their conversation and suggests a solution to Isabella. He tells her that she should agree to Angelo’s proposal but that he, the Duke, will try to persuade Mariana, who is the rejected fiancée of Angelo, to take Isabella’s place and sleep with Angelo instead.

Mariana agrees to the deception but, even though Angelo believes that he has shared Isabella’s bed, he still pursues his intention to have Claudio executed…