Vincentio Duke of Vienna appoints two Deputies to administer justice in his absence. Angelo the younger is to be assisted by Escalus,the elder.

On the streets of the city, the fop Lucio and two gentlemen are entertained by prostitutes. The Madam, Mistress Overdone, reports that the new deputy has imposed the death penalty on their friend Claudio for premarital sex. Lucio is asked to get Claudio’s sister, Isabella, to appeal for his life.

The Duke explains that he has appointed Angelo to enforce the law because the Duke himself has been too lenient and the people would resent his cracking down. He also wants to test Angelo. He disguises himself as a Friar to observe what happens.

Lucio approaches Isabella while she meets with the Mother Superior of the nunnery she hopes to enter. Reluctantly she agrees to her brother’s request.

Angelo refuses to lighten the sentence despite Escalus’ suggestion that he too might have behaved like Claudio given the chance. At first Angelo also rejects Isabella’s pleas, but after she presses them, he conceives a passion for her and invites her to return.

The disguised Duke enters the prison and administers confession to Julietta who expresses sincere repentance.

A conflicted Angelo propositions Isabella. She must yield her body to him in return for the release of her brother. Whe she threatens to expose him, he threatens to have Claudio tortured to death unless she yields. She responds "more than our brother is our chastity."

Still disguised, the Duke convinces Claudio not to fear death on the eve of his execution. When Isabella visits and tells Claudio of Angelo’s proposition, he begs her to comply to spare his life. Furiously she rejects him and storms out. The disguised Duke overhears their exchange and tells Claudio that Angelo was only testing his sister and that Claudio must die. Claudio accedes and leaves. The Friar-Duke offers Isabella a way out by proposing a bed trick on Angelo--substituting the deputy’s former fiancee, Mariana, for Isabella at the nighttime rendezvous Angelo has demanded in

return for Claudio’s pardon. Isabella accepts.

The Friar-Duke meets Lucio, who tells him that the Duke was given to lechery and would never impose harsh penalties for it. Escalus sends Mistress Overdone and her pimp Pompey to prison.

Still grieving for her lost love Angelo, Mariana is introduced to Isabella by the disguised Duke, and the three work out the details of the bed trick substitution.

Friar-Duke expects news of Claudio’s pardon. Instead, he hears a message from Angelo demanding Claudio’s head, earlier than scheduled. Friar-Duke and the Provost plot to substitute the head of a convicted murderer, Barnardine, for Claudio’s and send it to Angelo.

Drunken Barnardine refuses to prepare for death. The Provost supplies the dead pirate Ragozine’s head. Friar-Duke meets Isabella but does not tell her about the head trick, instead suggesting that she call out for justice and expose Angelo when the Duke himself returns next day.

At the city gate, Isabella publically accuses Angelo of murder, hypocrisy and violating a virgin, omitting mention of the bed trick substitution. The Duke acts as if she is mad, lets Angelo sit in judgment of her, and calls for the arrest of the Friar who egged her on. Mariana contradicts Isabella, insisting that she was the one Angelo slept with. Angelo angrily denies any recent contact with her. The Duke departs suddenly and returns again disguised as Friar who condemns the absent Duke for allowing Angelo to stand in judgement. Angelo and Escalus call for the Friar’s arrest, and when he resists, command Lucio to apprehend him. In the ensuing scuffle, the Friar is unmasked. Angelo admits his guilt and requests the death penalty. The Duke requires he marry Mariana to make her a rich widow first. Mariana begs for his life. The Duke denies her request until Isabella joins her in pleading that Angelo be forgiven. Now the Duke has Claudio and Julietta brought in. He pardons them and Angelo and Lucio, who is required to marry the mother of his child. Finally he asks Isabella to pardon him and to share his life. The final decision is left to her.