The Duke of Vienna tells his deputy, a wise lord called Escalus, that he is leaving the country on business. In his absence, he is handing power to Angelo, a less experienced man than Escalus. He asks Escalus what he thinks of Angelo and Escalus says he is more worthy than 'any in Vienna'. Angelo arrives and asks the duke for more of a test of his 'mettle' before he’s given so much power, but the duke insists and leaves secretly, without announcing to his people that he is going or why.

In the streets of Vienna, Mistress Overdone, a brothel-keeper, tells Lucio that his friend Claudio has been arrested for getting his fiancée pregnant. Pompey, who works for Overdone, brings news that Angelo is to have all the brothels in the city 'plucked down'. Overdone worries about her future and Pompey reassures her. They see Claudio being led past to prison. Claudio tells Lucio that Juliet 'is fast' his wife, but under these new strict laws, he could be executed for her pregnancy. Claudio asks Lucio to persuade his sister, Isabella, to plead for his life. Isabella is chaste and about to enter a convent and he has 'great hope' she will move Angelo.

The duke asks Friar Thomas for 'secret harbour' in the monastery. He admits that he has allowed the law to slip and that the city has become corrupt. He asks to be disguised as a friar so he can return to Vienna and secretly watch how Angelo copes with his new power.

At the convent, Isabella questions a nun called Francisca about the nuns’ religious life. Isabella says she wants a life of 'more strict restraint'. Lucio arrives with news of Claudio’s fate, describing Angelo as a man 'whose blood / Is very snow-broth' but urging Isabella to use the 'power' she has as a woman to move him. Isabella is less sure of this 'power' but agrees to meet Angelo.

Escalus tries to persuade Angelo to spare Claudio’s life, reminding him that he may have 'erred' in the same way. But Angelo says he will not 'make a scarecrow of the law' and that it is 'one thing to be tempted' but 'another thing to fall'. Elbow, a foolish but well-meaning constable, enters with Pompey and Froth as prisoners. Angelo quickly tires of their squabbling and leaves, handing the case to Escalus who deals with the men firmly but fairly.

The Provost tries to persuade Angelo to spare Claudio’s life but Angelo tells him to do his 'office' or lose his job. The Provost asks what to do with 'the groaning Juliet', Claudio’s fiancée, who is close to giving birth. Angelo orders her to be taken to a different prison. Isabella and Lucio arrive. Encouraged by Lucio, Isabella tells Angelo that she hates Claudio’s crime but begs for his life. Angelo refuses. Before leaving, Isabella offers Angelo a 'bribe' - to pray for his soul. Angelo tells her to come back tomorrow. Left alone, Angelo reveals in a soliloquy that he is sexually attracted to her.

Disguised as a friar, the duke visits Juliet in prison. He asks her if she repents, which she does. The duke tells her that her sin is 'of heavier kind' than Claudio’s and that he is about to visit Claudio with news of his execution. Juliet is in despair.

Alone, Angelo struggles with his feelings for Isabella. When she arrives, he tells her that he’ll save Claudio’s life if she sleeps with him. Isabella says she would rather die than give her body 'up to shame' and threatens to 'tell the world aloud' he is a hypocrite. Angelo says that no one would believe her. Left alone, Isabella realises he is right and goes to tell her brother what has happened.

The duke, in his disguise as a friar, visits Claudio and prepares him for his execution. Claudio says he is ready and that in 'seeking death' he will 'find life'. When Isabella arrives, the duke hides and listens as she tells Claudio about Angelo’s offer. At first, Claudio is horrified but then admits he is scared to die and suggests that a sin 'to save a brother’s life' would become a virtue. Isabella is disgusted and says he is better off dead. The disguised duke interrupts and speaks to Isabella privately, with a plan to save her virtue and her brother. They will trick Angelo into thinking he’s slept with Isabella but really they will swap her in the dark with his ex-fiancee, Mariana, who still loves him.

Pompey is brought to prison for being a bawd. He asks Lucio to pay his bail but Lucio refuses. Lucio speaks to the friar, not recognising the duke in disguise. Lucio criticises the duke for leaving Vienna and putting Angelo in charge. He calls the duke a drunk who wouldn’t punish lechery as strictly because he is guilty of it himself. The disguised duke can’t defend himself but threatens to report Lucio when the duke ‘returns’. Escalus brings Mistress Overdone to the prison for running a brothel. She accuses Lucio of giving 'information against' her and reveals that he has a child with a woman who he promised to marry. In a soliloquy, the duke reveals his anger for Angelo who 'weeds' the vice in the city whilst letting his own sins grow.

In a moated grange, Isabella and the disguised duke approach Mariana, who is still grieving for her relationship with Angelo. Isabella tells the duke that Angelo has given her two keys to a private garden where she is to meet him that night. The duke tells Isabella to explain the plan to Mariana. When they return, the disguised duke tells Mariana that sleeping with her ex-fiancé Angelo is not a sin because he is still her 'husband on a precontract'.

The Provost gives the job of executioner’s assistant to Pompey who is delighted to escape his 'full time of imprisonment'. Angelo believes that he has slept with Isabella but breaks his promise to her and orders Claudio’s head to be sent to him that afternoon. The disguised duke realises Angelo has broken his word and arranges for another man’s head to be sent to Angelo instead.

Left alone, the duke reveals that he intends to write to Angelo, announcing his public return to Vienna. Isabella arrives to ask about Claudio’s pardon but the disguised duke lies and tells her that Claudio has been executed. Isabella vows to go to Angelo and 'pluck out his eyes'. He tells her the duke is returning tomorrow and she and Mariana must accuse Angelo in front of him. Lucio sympathises with Isabella and criticises the duke 'of dark corners' again, unaware that he is actually speaking to him.

Angelo and Escalus discuss the duke’s 'uneven and distracted' instructions. They have been told to meet him at the city gates and that any citizens who want 'redress of justice'  can show 'their petition in the street'. In a soliloquy, Angelo reveals that he is relying on Isabella’s 'tender shame' to keep her quiet.

The duke meets Friar Peter without wearing his disguise. He gives him letters for the Provost who knows his 'purpose' and his 'plot'.

Isabella isn’t happy with the complicated plan to reveal Angelo. She wants to be the one to publicly accuse him but accepts that it’s Mariana’s job. Mariana reminds her that this is the plan of 'the friar' and they should 'be ruled by him'. Friar Peter arrives to take them to meet the duke.

Everyone is gathered at the city gates as the duke arrives. Isabella accuses Angelo, calling him a 'virgin-violator'. The duke pretends not to believe her and has her arrested. Mariana is brought in and reveals the bed-trick. Angelo admits he knows Mariana but says both women are mad and must be 'instruments of some mightier member' who plots against him. Isabella mentions ‘Friar Lodowick’ and Lucio immediately accuses the 'meddling friar' of slandering the duke. The duke sends for Friar Lodowick and then leaves for a while while Escalus questions Lucio further. When the duke re-enters, disguised as Friar Lodowick, Lucio tears off his hood, revealing the duke himself. Angelo confesses his 'shame' and begs for death but the duke sends him away to marry Mariana.

The duke apologises to Isabella for not preventing Claudio’s death. Angelo and Mariana are brought back in, married, and the duke orders Angelo’s execution to make things even: 'measure still for measure'. Mariana asks Isabella to plead for her husband’s life and Isabella agrees, saying that 'sincerity govern’d' Angelo until he met her. The duke then reunites Isabella with Claudio, who is still alive, and asks her to marry him. The duke pardons Angelo and orders Lucio to marry the woman he made pregnant. The duke offers himself as a husband to Isabella again. Isabella does not reply.