The Duke of Vienna sets the play in motion by temporarily abdicating his throne in favor of his deputy Angelo. He wishes to purge the city of its vices and restore the enforcement of longstanding laws against various crimes. However, he fears being seen as a tyrant if he carries out these reforms himself. While Angelo is implementing his own strict brand of justice, the Duke visits the city disguised as a friar. Although he is responsible for setting things right at the end of the play, the Duke is a morally ambiguous character. He evidently likes keeping his subjects in the dark, as he gives conflicting information to his highest officers and later deceives Isabella about Claudio's death. He is also somewhat vain, as evidenced by his harsh treatment of the slanderous Lucio at the end of the play.
When the Duke suddenly leaves Vienna, Angelo is appointed to rule until his return. He immediately begins to tighten up law enforcement, which he sees as having been excessively lax during the Duke's rule. His main initiative is the revival of a long-forgotten law making extramarital sex a crime punishable by death. At first Angelo believes himself virtuous and insists on holding others to the same high standard. This moral self-certainty is almost immediately shattered when Angelo finds himself lusting after Isabella and abusing his official powers to extort sexual favors from her. From this point on Angelo is ashamed of his own conduct and experiences a painful sense of inner conflict, although not enough to change his behavior. He is almost relieved when his crimes are exposed at the end of the play.
Isabella plays a much more active role in the plot than does her brother, Claudio. When she first appears onstage, she is a novice—a nun-in-training—who is about to renounce her earthly life and enter a convent. Claudio's predicament draws her back into public life, where she uses her knack for persuasive speech to plead on her brother's behalf. Strictly virtuous in her own conduct, Isabella refuses to sleep with Angelo even to save her brother's life. For her, giving in to such a request entails a moral death far worse than physical suffering. At the play's end, it remains unclear whether she accepts the Duke's offer of marriage or returns to the convent.
Worldlier than his sister, Claudio finds himself at the mercy of Viennese law when it becomes evident that his fiancée, Juliet, is pregnant. He is at first appalled by the choice Isabella is forced to make and supports her in her refusal to sleep with Angelo. However, moments later he tries to persuade his sister to change her mind. Saving his life, he reasons, is more than sufficient reason to abandon her virginity. Although the play's action centers on Claudio, he is more a victim than an active protagonist. He spends much of the play in a Viennese prison cell, unaware of the Duke's plot to rescue him.
Escalus is one of Vienna's leading judges and a man highly respected by the Duke. Despite having more experience than Angelo, he takes a back seat to his colleague when the Duke leaves the two in charge of Vienna. Like Angelo, he is puzzled by the Duke's behavior but, as a loyal subject, agrees to do as asked. During the Duke's absence, Escalus is ordered merely to advise, not to challenge Angelo's decisions. He accepts this restriction for the most part when Angelo is present. When he has the chance to interact with citizens directly, however, Escalus shows himself much more lenient than Angelo. He even lets a few suspected criminals off with a warning—something Angelo would never dream of doing.
Angelo abandoned Mariana five years ago because of her family's inability to pay the dowry. Despite this act of callousness, she continues to long for him, listening to sad songs and thinking about what might have been. When the Duke devises a plan to trick Angelo, Mariana willingly goes along with the ruse. By sleeping with Angelo, she will consummate the marriage he agreed to but failed to follow through on. Her loyalty to the undeserving Angelo is a mystery to the other characters.