The city of Vienna is in trouble morally and spiritually. The Duke of Vienna decides to go in disguise and see just how bad things have become. He also wants to clean up the city but does not want to use his authority. He asks Lord Angelo to be in charge while he—the Duke—is away “traveling”, and instructs him to enforce strict moral laws.
Lord Angelo takes great delight in carrying these orders out, and ends up sentencing Claudio to death for impregnating his fiance Juliet before they were married. Lucio finds out about the sentence and sends for Claudio’s sister Isabella, who is so spiritually pure that she is planning to enter a convent. She comes back to the city and pleads with Angelo for leniency for her brother. Angelo agrees, but only if she will sleep with him. Appalled, she refuses, despite Claudio’s pleas for her to agree to the terms to save his life.
Serving as a friar for the court, the Duke comes up with a plan to free Claudio, save Isabella’s dignity, and catch Angelo. Angelo was once betrothed to a woman named Mariana, but he broke off their contract when her dowry was lost at sea. The Duke tells Isabella to agree to Angelo’s proposal, but that night, Mariana would take her place and Angelo would be forced to marry Mariana and pardon Claudio. The plan works perfectly, but in the morning, Angelo does not pardon Claudio; instead, he asks his head be delivered to him after the execution. The Duke arranges for the head of another prisoner being executed that morning to be delivered to Angelo instead. The Duke also tells Isabella to make sure she brings her grievance to the Duke when he returns to the city from his trip.
The Duke, out of his friar disguise, arrives in the city and is greeted by Isabella who tells him everything about Angelo. Angelo denies the allegations and the Duke pretends to believe him, to the dismay of Isabella and Mariana. He sends for the friar and leaves the room, coming back in his disguise to defend the women. After Angelo accuses the friar of lying, the Duke reveals his true identity and orders Angelo to marry Mariana. He then reveals that Claudio is still alive, and orders him to marry Juliet. He saves his final marriage business for last, and proposes to Isabella.