A duke leaves his kingdom in charge of a precise and justice-obsessed man, who then sets out to punish all according to the very letter of the law. A man stands condemned to die for impregnating his fiance. A young nun, un-worldly and pure, leaves her cloister to save her brother. Such is the state of the London-like Vienna at the start of Measure for Measure, a play that poses difficulties in genre classification and interpretation.
A comedy usually follows the line of A Midsummer Night's Dream--characters leave a world for an enchanted place, learn and change and grow, and then return different people. Inevitably, marriage follows. Measure for Measure ends in marriages, but the marriages are more punishment than anything else. Characters change and grow, but it seems like they cannot act on what they've learned. Tonally, the play is in not comedic, with its talk of deflowering nuns, lying, and death it is a dark play. Overall, Measure for Measure seems to represent an aborted comedy, a demi-tragedy that leaves the reader wondering how to interpret the characters and events.
For example, there are two ways to view the character of the Duke. He disguises himself as a friar after leaving Angelo in charge of ruling his kingdom and travels around Vienna in the office of a friar. One way to see him is a divine force that brings enlightenment to the characters through his actions. However, he could also just as easily be a manipulative, voyeuristic liar, who twists situations and those around him to his own sinister designs. I tend to see him as a mixture, but an overall disagreeable figure.
I saw the Duke as a liar who manipulated Isabella to his own designs, who later asks a nun to marry him, who wants to believe that he does his actions for the good of his land, but is actually self-serving and cowardly. He runs away from his responsibility in administering justice because he wants his people to love him. At the end of the play, he forces Lucio (a figure of imagination and light, although a bit of a rascal) to marry a prostitute and then be hanged afterward because he was critical of the Duke. But others can just as easily see him as a more benevolent figure, burdened with the conflict between absolute law and wishing to administer mercy.
The entire play follows along this lines. Shakespeare offers us several ambigious circumstances and characters, and leaves it to the reader to interpret them or the play's director to make decisions on how to act the parts. Shakespeare cleverly demonstrates that often there are different ways to interpret a situation and often different perspectives on the telling of a story, a theme that runs constant through his plays.