The Duke of Vienna, Vincentio, leaves the city on a diplomatic mission. While he is away, he places Vienna in the care of his subordinate, Angelo, who decides to tighten the city's anti-fornication laws. A young nobleman, Claudio, has sex with his wife-to-be, Juliet, before they are married, and Angelo decides to execute him as punishment. Claudio's sister Isabella, a nun, tries to save him, but Angelo will only save Claudio if she agrees to sleep with Angelo, in exchange. At first, Isabella refuses -- much to her brother's horror. In fact, the Duke has not really left the city, and instead has disguised himself as a friar, in order to see what would happen to the city in his absence. As the friar, the Duke makes Angelo think that he has sex with Isabella, when really he has sex with Mariana,a woman to whom he was betrothed, but who he abandoned. Second, when Angelo asks for Claudio's head, the Duke provides the head of a lookalike. When the Duke decides reveals his true identity at the last minute, Isabella and Mariana both petition him to punish Angelo for his misconduct. Angelo tries to weasel out of things by blaming his actions on the friar, not knowing that the friar was, in fact, the Duke -- in disguise. The Duke reveals himself to be the friar, and Angelo is punished by being forced to marry Mariana. Likewise, Claudio's friend Lucio, for slandering the Duke and the friar, who were the same man, is forced to marry a whore.