Measure for Measure plays on the themes of corruption and purity. The dichotomy between the two are discussed and examined. Unlike some other "problem plays" Measure for Measure does not end in tragedy. Therefore it is most commonly classified as a comedy despite it's questionable tone.
The play begins in Vienna, as the Duke of that city, Vincentio announces publicly that he plans to leave for a diplomatic engagement. In his wake he leaves the strict enforcer judge Angelo in charge.
The next scene finds a group of soldiers in Vienna hoping lustily for war with Hungary. The owner of a local whorehouse that they all frequent, Mistress Overdone, appears on the street telling them that one man who impregnated Madam Juliet will be beheaded as a punishment in a few days. She swears he is a good man. His name is Signor Claudio. A friend of Claudio's, Lucio, rushes off stage.
The bum Pompey appears, one of Shakespeare's most celebrated characters. He works as a pimp for the Mistress Overdone but doesn't advertise that face, instead appearing as a bartender. He defends Claudio and calls the case against him confused. He informs Mistress Overdone of the strict Angelo's proclamation that all houses of prostitution will be abolished in Vienna, to which she is distraught. She worries for her own fate but Pompey consoles her, telling her not to fear and that society will still have a place for her.
The case against Claudio becomes clear when he is led through the streets past Pompey and Madame Overdone on his way to prison. He had married Juliet but hadn't completed all the technicalities so the marriage wasn't complete when she became pregnant. When Angelo stepped in as defacto ruler he instituted a law making fornication a death offense. Claudio was sentenced to death. His sister Isabella, a new nun, pleads for his life on his friend's Lucio's request to Angelo.
After two scenes of Isabella's protestation its clear Angelo has desire for her. He finally offers her a deal: he will spare Claudio's life in exchange for taking Isabella's virginity. Isabella adamantly refuses and threatens to tell everyone about his absurd offer. As a nun she believes her virginity is worth more than Claudio's earthly life and that giving it to Angelo will doom both her immortal soul and Claudio's.
The Duke in disguise as a friar named Lodowick befriends Isabella and helps her set up a trick. She agrees to submit to Angelo if the deed is done in complete darkness and silence. Angelo's former betrothed Mariana, who he has left because her dowry was lost at sea, takes Isabella's place. Angelo is none the wiser, and by sleeping with Mariana he actually consummates their marriage he wanted out of.
Even though Angelo believes he has enjoyed Isabella, he doesn't honor his word to save Claudio. He asks to see Claudio's head. The Duke sends the head of a pirate who died recently of illness and resembles Claudio.
The Duke Vincentio stages his 'return' to the city and hears claims from Isabella and Mariana on Angelo's cruelties. It appears that the Friar, the Duke in disguise, will be blamed for the offenses and that Angelo will sentence him. The Duke appears back in his disguise as the Friar but then reveals his true identity. He first decides Angelo should be executed but then presses him to marry Mariana and give her his estate as her dowry.
The play ends with the Duke proposing his hand in marriage to Isabella. Claudio's friend Lucio is punished for slander against the Duke by being forced into an unwanted marriage with Kate Keepdown, a local prostitute.