Context: Duke Vincentio of Vienna, a too-kindly man, realizes that lax law enforcement in the city must be corrected. He turns over the cares of state to his deputy, Angelo, and departs. Angelo revises an old statute which imposes the death penalty for fornication. The first victim, who is to be an example, is a young noble, Claudio, who, engaged to Juliet upon a true contract of marriage, has got his fiancée with child. Claudio's sister, Isabella, a beautiful novice in a convent, pleads with Angelo for Claudio's life. Angelo will spare Claudio if Isabella gives herself to him. The duke, returned, learns of these events. Disguised as Friar Lodowick, he tells Isabella how she may save her brother without sacrificing herself. Five years earlier, he reveals, Angelo had made a similar contract to marry a noble lady, Mariana, but jilted her because she could not produce her dowry. Isabella is to agree to an assignation with Angelo, and then Mariana will, in the dark, take her place. The plan succeeds. Now the duke, to prepare the final exposure of Angelo, questions the veiled Mariana.