In the Duke's absence from Vienna, his strict deputy Angelo revives an ancient law forbidding sex outside marriage. The young Claudio, whose fiancée is pregnant, is condemned to death by the law. His sister Isabella, soon to become a nun, pleads with Lord Angelo for her brother's life. But her purity so excites Angelo that he offers her a monstrous bargain - he will save Claudio if Isabella will visit him that night.