The play takes place in Vienna that is described as a completely spoiled city, full of promiscuous people, sexually-transmitted disease and perverted sex. The Duke of Vienna is deeply disappointed with this state of affairs and we see him lamenting and complaining that the city is too spoiled and the people need to be put in line immediately. But he doesn’t want to be the one who will sign the unpopular laws, so he decides to take a vacation and leave the city, appointing his second-in-command, a man named Angelo, as a temporary city ruler, so that he would be the one to blame. Angelo is a bit confused to take such a great responsibility and hardest task, but he is honored to rule Vienna in Duke’s absence. Duke leaves, wishing Angelo good luck in returning morals and discipline to the city inhabitants.
The narrative shifts to the streets of Vienna and we have a brief glimpse of the people there (and start to understand why the Duke wants to enforce moral laws). We see Lucio, a rich man, who prefers libertarian way of life, socializes with two other men. They exchange the rumors about their dirty “feats”, mostly connected with visiting brothels and sleeping around. Mistress Overdone comes with the news that Claudio, their friend who is generally considered a good man, has been jailed for sleeping with his fiancée before the wedding and impregnating her. We see Claudio who is escorted to the prison by the city guardsmen and lamenting that he isn’t guilty, because Juliet, the pregnant woman, was almost his wife and one formal document about their wedding should not make the difference. Seeing Lucio nearby he asks him to bring news about his imprisonment to his sister, Isabella, who is living in the monastery. Isabella was always very intelligent and was a good friend of Angelo once, so now Claudio hopes that she can use her wit and connection to save him from the jail.
In the meantime, the Duke isn’t intended to leave the city for real. He comes to his friend, Friar Thomas and asks him to hide him for a while. The Duke wants to see how Angelo will implement the new laws and what impact will they have on the city population. He complains to Friar Thomas that the people wouldn’t understand that the laws were made for their own good and would be angry at Duke for signing it. But still he is completely sure that it has to be done, the only his doubt is if Angelo has enough courage to put an end to the citizens’ amoral behaviour. The Duke’s absence is also a secret test for Angelo: he has to prove that he has enough intelligence and willpower to complete such a task.
We see Lucio coming to the monastery to see Isabella. He tells her the latest news about Vienna, that Angelo now is a temporary ruler who implemented very strict moral laws right after his appointment. He also tells that her brother Claudio became one of the first victims of these rules, because he impregnated his wife-to-be Juliet. Moreover, he is now sentenced to death and his trial should be a public warning for everyone else. Isabella is shocked by the news, but she fears that she is powerless in this case. The only thing she can do is to meet with Angelo and try her best to beg for Claudio’s life and freedom.
Meanwhile Angelo continues threatening the citizens with the heavy hand of law. His counselor, Escalus agrees with him, but cautiously warns Angelo that his actions may easily slip from the territory of justice to the realm of pointless cruelty and mercilessness. Escalus faithfully helps Angelo in his crusade against the sexual liberties, though he has sincere compassion towards Claudio (he is one of the less guilty amongst the prisoners), but he is afraid that sparing him will show that the new laws aren’t as strict as they were presented to the crowd. When they finish their talk, the Provost comes to Angelo to ask if it is really necessary to execute Claudio - his sin isn’t as horrifying, moreover, he still intends to marry Juliet. But Angelo is adamant. The next visitor after the Provost is Isabella, who begs for her brother’s life, reminding Angelo about the mercy and moderation, hoping that his wisdom will allow him to divide the truly grave crimes from sins made because of excessive emotions, love and passion, that can be still reverted and repented.
In the meantime, the Duke in disguise of a friar goes to the prison, pretending that he is there to minister the condemned prisoners. But his real intention is to learn how many prisoners are in jail and why they are there, so that he can make conclusions about Angelo’s efficiency. The Provost escorts him to Claudio, telling him about the cause of the unfortunate man. The Duke then comes to Juliet to hear her opinion. He asks her if she regrets her sin and she says that she does. But from the sincerity of the girl, the Duke learns that she and Claudio are in love for real and she gave her consent to him to sleep with her. Playing his role as a friar, the Duke tries to comfort Juliet, saying that God will have mercy on Claudio’s soul and leaves.
When Isabella leaves, Angelo finds that he himself isn’t flawless: he lusts for her and this feeling is almost unbearable. When Isabella comes again to plead Angelo once more, he can’t hold his desire anymore. He starts from hypothetical question, if Isabella is ready to sacrifice her honor and virginity - everything that values so much for a woman from the monastery - to save her brother. Isabella angrily says that she will never do such a thing. Enraged by her answer, Angelo openly tells her that either she sleeps with him or Claudio will be executed. Isabella, though she is shocked with such a villainy, stays adamant in her decision.
The Duke, still in disguise, returns to Claudio in his cell. Claudio’s hope to survive through everything is still alive, but the Duke (still intended to punish him for his deed a little bit) says him to prepare for death: either he will die in peace or the unexpected freedom will be much sweeter. Suddenly Isabella, who has just finished her negotiations with Antonio, enters and the Duke exits. But he stays nearby, listening to their conversation. Isabella angrily tells Claudio about Angelo’s conditions, that to save his life she has to give up to Angelo, a hypocrite who is ready to kill Claudio for making love to the woman that he should marry. Claudio, in disgust, says that she should never do this and he’ll rather die to protect her honor, but soon, when he thinks about painful and shameful death that awaits him and the fate of Juliet and their baby, who also will be forever scorned - and begs his sister to agree to Angelo’s condition and sleep with him. Isabella again experiences the same shock, angrily asking her brother if he values her so low. While they are arguing, the Duke enters again, asking to speak with each one in private. He tells Claudio that Angelo’s conditions were just a test of Isabella’s morals and determination to stay pure - the test she passed brilliantly. Then he walks out with Isabella and says that he knows about Angelo’s immorality and he has a plan to save Isabella’s honor, free her brother and punish Angelo simultaneously. The Duke tells Isabella the story of Mariana, the girl who was engaged to Angelo, but abandoned by him when he realized that she has lost her dowry due to unfortunate circumstances. The plan is that Isabella goes to Angelo and says that she is ready to agree to his condition. But when Angelo will invite her to his bedroom, it will be Mariana who enters it and the Duke (as a friar) will be the witness of it. So, in the morning Angelo would be obliged to marry Mariana, while Claudio would be already free and Isabella intact. Isabella gladly agrees to the plan and returns to Angelo to says that she gives up, while the Duke finds Mariana and explains her part to the woman. Lucio, who also happens to be nearby, asks the Duke (as a friar) where the Duke can be, so he can send him a formal complaint. The Duke answers sarcastically that he doesn’t know. Lucio laments that if the Duke was around he would never allow Angelo to wreak havoc in Vienna with such strict laws, because the Duke himself is a jovial sinner, a fan of love affairs, drinking and other delights of flesh, so he would have mercy on them all. The Duke is not amused with such a revelation.
Meanwhile, Escalus and the Provost take Mistress Overdone to the custody. Lucio accused her of running a brothel, because she, in her turn, called him out for impregnating one of her prostitutes, Kate Keepdown and refusing to support their baby. Mistress Overdone is proclaimed guilty and goes to prison, but Escalus promises that Lucio will also suffer for his sin.
The Duke comes to Mariana and explains her his plan. Isabella also comes to him and says that she talked to Angelo and agreed to his demands. Isabella talks to Mariana, persuading her to take her place. Mariana still doubts, asking the friar if she commits a sin, but the Duke assures her that there will be no sin in her actions.
This time in prison the Provost shows to Claudio the order to execute him. The Duke, still in disguise of a friar, enters next and prays with despaired Claudio, saying that there is always hope for a miracle and Claudio needs to trust God and believe in Him. The Provost brings the message: the Duke is sure that it is pardon, because Isabella has agreed to sleep with Angelo, but, strangely, it is an order to execute Claudio immediately, no matter what and regardless of any circumstances. Shocked by Angelo’s behaviour, the Duke speaks to Provost, offering him to send to Angelo the head of another condemned prisoner named Barnardine, so they will win some time for Claudio. The Provost doesn’t want to disobey his temporary ruler, but finally he admits that he also doesn’t think that Claudio deserves death and agrees to help. The Provost orders to bring Barnardine, but he barely stands on his feet, claiming that he is drunk and wants to sleep. The Duke, as a friar, says that he can’t allow a prisoner to die with his soul unprepared for the justice of God. Luckily it appears that another prisoner has just died of natural causes in his cell, so they can use his head as a proof for Angelo. They cut the head off the dead prisoner, playing all the execution right before Isabella comes to the prison, asking if the pardon was already granted. But the Duke, still faithful to his plan, tells her that her brother is dead and Angelo signed no pardon for him. He tells Isabella that the Duke will return tomorrow and will be ready to listen to her grief and anger. Lucio (who still isn’t in jail) enters and, after he understands what happened, tells Isabella his condolences and hopes that the Duke will punish Angelo for such a mischief.
Angelo receives a letter from Duke in which he announces his return. Angelo decides to show off his work and orders to tell everyone that Duke will be here soon and will be ready to listen to the praises and complaints about Angelo’s work as a city ruler. He is sure that the citizens themselves will prove Angelo’s competence. The only thing he is afraid of is Isabella’s complaint that may cause some serious troubles. But he is sure that she doesn’t know about Claudio’s execution yet, so she will keep silent for the sake of her brother. Also, it will be a great shame for her, a woman from the monastery, to say aloud that she slept with a man. Angelo thinks about Claudio again, admitting that his sin wasn’t horrible enough to execute him outright.
Isabella and Mariana (who spent the night with Angelo instead of Isabella) are preparing their complaints to the Duke. Friar Peter - the real friar - comes to them and asks them to hurry because the Duke will soon appear near the gates. Isabella and Mariana come to the gates and join the huge crowd of other inhabitants of Vienna. Soon the Duke enters the gate. At first, he greets Angelo and Escalus, saying that he heard they were doing an excellent job and purified Vienna. He considers that Angelo should be rewarded and praised, but he still wants to listen to the city population just to confirm his opinion about Angelo and Escalus. When he comes to the people, Isabella is the first to speak. She accuses Angelo of being a mischievous hypocrite and a “virgin-violator”, adding that he can look as sweet as he pleases, but inside he is a completely different person and she demands an investigation of her case. The Duke allows Isabella to tell everything. He pretends that he doesn’t believe her at first, shocked by the accusations and saying that such a noble and pure man as Angelo is incapable of committing such an ugly crime. Friar Peter, though, asks Marianna to come and speak as a witness. She tells her own story about her engagement with Angelo and his abandonment when Angelo learned she wasn’t a rich heiress anymore. She also says that she slept with Angelo to save Isabella’s honor and her brother’s life and gave her virginity to him. Cornered, Angelo admits breaking the engagement, but he claims that he was justified in doing so, because it is scandalous to marry a girl without a dowry. The “investigation” continues. The Duke leaves “in business” leaving Angelo as a judge in his own case. Meanwhile Friar Peter brings another witness - Friar Ludovico (the Duke in disguise). When Friar Ludovico starts to speak, Lucio, who is also among the rest of the people, gets angry at him and pulls off his hood. Now everyone sees that Friar Ludovico is actually the Duke.
Now Angelo has no options than to confess and ask the Duke for mercy. The first order of the Duke for him is to immediately marry Mariana. Friar Peter takes them away and soon they return as husband and wife. Then the Duke says that Angelo still ordered to execute Claudio, despite, as he thought, sleeping with Isabella. So, he should also die for being much worse sinner than Claudio was. Shocked Marianna pleads for mercy for her husband - despite all his wrongdoings she still loves Angelo. Isabella can’t stand seeing her tears and joins her plea. Finally, the Duke softens and lets Angelo live. He also proclaims that Claudio is still alive, and concealing this fact was just a part of his plan. Claudio is released and joins the crowd. Seeing Isabella’s gratitude, the Duke makes her a proposal that is suddenly accepted due to her euphoria. The last judgement of the Duke is to order Lucio to marry the prostitute he impregnated, Kate - as a punishment for his sin and trying to hit the Duke, even in disguise. The Duke also says that Claudio is pardoned, he was punished harshly enough and now can marry his beloved Juliet. He also wishes all the best to Angelo and Marianna (at least she is happy with her marriage) and calls Isabella to him to prepare their marriage.