Shakespeare’s Measure for Measure centers on the fate of Claudio, who is arrested by Lord Angelo, the temporary leader of Vienna. Angelo has been left in charge by the Duke, who pretends to leave town but instead dresses as a friar to observe what goes 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 he 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 marriage. 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 to relent, but he also suggests there might be some way to change his mind. He propositions her, saying that if she agrees to have sex with him, then he will let Claudio live. Isabella is shocked and immediately refuses. Her brother initially agrees with her decision, but then he changes his mind and asks her to sacrifice herself to save him. 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 Angelo, but then Mariana will go in her place. The next morning, Angelo will pardon Claudio, and, according to the very law he’s sworn to enforce, he’ll be forced to marry Mariana.

Everything goes according to plan, except that Angelo does not pardon Claudio, fearing that the prisoner might seek revenge. But the Duke intervenes, arranging for the head of a different prisoner to be sent to Angelo. This substitute head having been delivered to Angelo, he believes that his orders have been carried out. Meanwhile, the Duke tells Isabella that her brother has been executed dead. He then instructs her to submit a complaint to the Duke. He is due to arrive shortly, at which time she must accuse Angelo of committing immoral and unlawful 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.