Vienna is teeming with brothels and loose morality, but the Duke doesn't want to use his authority to clean up the city. He departs, dressed as a friar, leaving his deputy, Angelo, in charge. Angelo is at first reluctant, but he soon starts to make changes.
Changing the law
A young man, Claudio, gets the girl he intends to marry, Juliet, pregnant. Under a forgotten old law that Angelo has just reinstated, Claudio is arrested and sentenced to death. Angelo revives other harsh laws and threatens to demolish all the brothels. Meanwhile, the Duke returns to the city in disguise.
Lucio, Froth and others (who regularly visit Mistress Overdone's brothel) are shocked to hear of Claudio's fate and agree that Lucio should visit Claudio's sister, Isabella, in her convent. They persuade her to leave the convent and appeal to Angelo to spare her brother.
Angelo offers to spare Claudio's life if Isabella will sleep with him, but she is about to become a nun and so refuses in order to protect her chastity.
Isabella visits her brother, Claudio, in prison and explains Angelo's offer and her refusal. The Duke, disguised as a friar, overhears their conversation. Claudio begs Isabella to submit to Angelo, but she refuses. The disguised Duke tells Claudio to prepare for certain death, but secretly tells Isabella of a plan to save Claudio's life without violating her vow of chastity. He reveals that Angelo has treated his ex-fiancee, Mariana, very badly and puts Isabella in touch with her.
Isabella and Mariana fool Angelo into sleeping with Mariana. Isabella agrees to submit to Angelo as long as it is in the dark and no one speaks but, instead of her, Mariana keeps the agreed meeting. Angelo will then have committed the same 'crime' as Claudio.
Meanwhile in prison...
Constable Elbow brings Pompey into the prison for being a pimp. Lucio meets the disguised Duke and pretends that he knows the Duke well, describing him as immoral. The Duke is outraged and determined to get revenge.
Mistress Overdone and a group of prostitutes are also imprisoned. Mistress Overdone discloses that Lucio has made one of her prostitutes pregnant and refused to marry her.
The Provost of the jail receives instructions from Angelo, demanding that Claudio be put to death at once. The Provost and Pompey devise a plan to execute another prisoner instead of Claudio, although the prisoner (Barnardine) is less keen on the idea of such a hasty death.
Stop reading if you don't want to know how it ends!
Luckily, another prisoner has just died of a fever, and his head is sent to Angelo instead of Claudio's. The disguised Duke tells Isabella that Claudio has been executed but says she will be able to speak to the Duke personally about Angelo. Lucio, believing he is still speaking to a friar, admits to the Duke that he made a prostitute pregnant.
Once the Duke's 'return' is announced, Angelo regrets executing Claudio. Isabella publicly denounces Angelo, but the Duke pretends to disbelieve her and has her arrested.
A friar (Peter) comes forward to support Isabella's story, and Mariana confirms that she was the one to sleep with Angelo and that he had earlier refused to marry her. The Duke (in disguise again) also confirms Isabella and Mariana's stories and, during a scuffle, is revealed to be the returned Duke.
On the Duke's orders, Friar Peter marries Mariana to Angelo, whom the Duke immediately condemns to death. The two women plead for Angelo's life, and the Duke relents. The Duke then has both Barnardine and Claudio brought before him, frees Barnardine and reunites Isabella with her brother. Lucio is forced to marry the prostitute he made pregnant, and is then condemned to death so that his widow can inherit his money.
The play ends when the Duke appears to propose marriage to Isabella, who does not answer him.