Shakespeare's Measure for Measure centers around the fate of Claudio, who is arrested by Lord Angelo, the temporary leader of Vienna. Angelo is left in charge by the Duke, who pretends to leave town but instead dresses as a friar to observe the goings-on in his absence. Angelo is strict, moralistic, and unwavering in his decision-making; he decides that there is too much freedom in Vienna and takes it upon himself to rid the city of brothels and unlawful sexual activity. Laws against these behaviors and institutions already exist, and Angelo simply decides to enforce them more strictly. Claudio is arrested for impregnating Juliet, his lover, before they were married. Although they were engaged and their sexual intercourse was consensual, Claudio is sentenced to death in order to serve as an example to the other Viennese citizens.
Isabella, Claudio's sister, is about to enter a nunnery when her brother is arrested. She is unfailingly virtuous, religious, and chaste. When she hears of her brother's arrest, she goes to Angelo to beg him for mercy. He refuses, but suggests that there might be some way to change his mind. When he propositions her, saying that he will let Claudio live if she agrees to have sexual intercourse with him, she is shocked and immediately refuses. Her brother agrees at first but then changes his mind. Isabella is left to contemplate a very important decision.
Isabella is, in a way, let off the hook when the Duke, dressed as a friar, intervenes. He tells her that Angelo's former lover, Mariana, was engaged to be married to him, but he abandoned her when she lost her dowry in a shipwreck. The Duke forms a plan by which Isabella will agree to have sex with the Angelo, but then Mariana will go in her place. The next morning, Angelo will pardon Claudio and be forced to marry Mariana according to the law.
Everything goes according to plan, except that Angelo does not pardon Claudio, fearing revenge. The provost and the Duke send him the head of a dead pirate, claiming that it belonged to Claudio, and Angelo believes that his orders were carried out. Isabella is told that her brother is dead, and that she should submit a complaint to the Duke, who is due to arrive shortly, accusing Angelo of immoral acts.
The Duke returns in his usual clothes, saying that he will hear all grievances immediately. Isabella tells her story, and the Duke pretends not to believe her. Eventually, the Duke reveals his dual identity, and everyone is forced to be honest. Angelo confesses to his misdeeds, Claudio is pardoned, and the Duke asks Isabella to marry him.