Play by William Shakespeare set in an indeterminate time in Vienna, Austria. Isaac Asimov says in Asimov's Guide To Shakespeare: "In this play, written in 1604, Shakespeare takes the opportunity to study the relationship between justice and mercy. He had done so in The Merchant of Venice, but there he had not been consistent. Portia had demanded mercy of Shylock, but when the tables were turned she did not show it. ... In this play Shakespeare carries through the notion of mercy to ultimate consistency, and in offering mercy to the villain makes many critics unhappy. In presenting an unpleasant situation so that the offering of mercy becomes hard indeed, more critics are made unhappy. The result is that Shakespeare's great play of mercy is usually considered one of his unpleasant comedies, like All's Well That Ends Well."

Basic Plot: Vienna's Duke has decided to enforce the city's laws on sexual morality. He appoints a lord, Angelo, to do this work and then tells everyone he's leaving town for a while (so that he can secretly hang around and see how enforcing these laws works out).

In the next scene, we find that Angelo is enforcing the laws by slating for execution Claudio, who is engaged to Juliet but got her pregnant while the dowry was being negotiated. Claudio's friend Lucio suggests getting Claudio's virtuous sister Isabella to plead his case to Angelo. Lucio goes to see Isabella (a novice in a convent) and she reluctantly agrees to speak for her brother, but she's not happy about what he did either.

After we see the trial of some people for being at a brothel, Isabella comes to plead for Claudio, but her disapproval shows through and she's not all that convincing. She does however, tell Angelo that mercy would befit him best. Angelo finds Isabella attractive and asks her to return the next morning. She does, and at that point Angelo says he'll release Claudio if Isabella will sleep with him. She refuses and leaves to tell Claudio; at first her brother agrees with her position but after thinking about dying, he asks if she won't sacrifice her virtue for him. She refuses and turns to go; the Duke, who, disguised as a friar, overheard her, tells her that Angelo was engaged to Mariana, but dumped her when her family lost its fortune. He suggests that they get Mariana to go, disguised as Isabella. Afterwards, they will reveal the truth and force Angelo to marry Mariana. Isabella agrees.

One of the brothel servants from the earlier trial comes back on stage, arrested again. He cries out to Lucio who he recognizes as a customer to help him, but Lucio just jokes. Lucio then talks to the friar he doesn't know is the Duke and says all sorts of slanders against the Duke. (In the next scene, the madam of the brothel says that Lucio has fathered a child on one of the women there.)

After Mariana has substituted for Isabella and slept with Angelo, he starts fearing discovery and decides to execute Claudio anyway so that no one will believe Isabella if she ever tells on him. He sends an order to the prison to have Claudio killed; the friar/Duke intercepts it and sends Angelo the head of a prisoner who died of disease. But when Isabella shows up, the friar/Duke tells her Claudio is dead, and she cries for revenge.

After this, the Duke returns to Vienna as himself. Isabella complains to the Duke about Angelo; Angelo denies it and the Duke pretends to believe him. Mariana comes in and corroborates the story; Angelo still denies and the Duke still pretends to believe him. The women talk of the friar who gave them the idea, and Lucio mischeviously adds that the friar was slandering the Duke (when actually Lucio was). At this point, the Duke leaves the room, comes back in friar's robes, and is arrested, at which point he pulls off his hood to reveal who the friar is.

Angelo confesses at this point and asks for death; the Duke says he must marry Mariana first. They go offstage while Isabella asks forgiveness for unknowingly treating the Duke as less than his station and is forgiven. Angelo comes back on stage, and first the Duke says he deserves the kind of justice he gave out. Mariana pleads for his life with both the Duke and Isabella; Isabella, though steaming for revenge at first for the brother she still thinks is dead, eventually agrees to forgive Angelo and also speaks for him to the Duke.

The Duke pardons Angelo, and then turns to Lucio, who he sentences to marry the prostitute who is the mother of his child, then to be whipped and hanged. Lucio "seems to be more dismayed at the disgrace of the marriage than at the rest," according to Asimov, and starts to make jokes. The Duke finally decides to be merciful to Lucio as well (canceling the whipping and hanging, but not the marriage) and in his last speech, indicates that he will marry Isabella.