The plot centres on Angelo, the righteous deputy empowered by the Duke of Vienna, to rule whilst he wanders about disguised as a friar to investigate the moral decay of his dukedom. Resorting to an archaic law against fornication to enforce his strict standards of morality, Angelo proceeds to condemn fornicators to death. One of these fornicators is Claudio , a young man who has had pre-marital sex with his fiancee.

When Claudio's virginal sister Isabella comes to plead for her brother's life, Angelo's lust is aroused and he uses his power to blackmail Isabella into his bed. It's her virtue or Claudio's life. She threatens to expose him but is told "who will believe you?" Fortunately the Duke overhears. Angelo ends up in the bed of Mariana , a woman from his undisclosed past, Claudio is allowed to live happily ever after with his fiancee, and the virtuous Isabella ends up the Duke's bride