The more I read of Shakespeare the more I think that the best way to think of him is as an experimental author and Measure for Measure is one of his more experimental plays. It is based on what was essentially a contemporary urban myth of a man who has been sentenced to die for having sex outside of wedlock and whose lover/sister is offered a chance at reprieve by the authority in charge in return for sex.
Shakespeare takes this myth and turns it into a comedy. That is in itself a bizarre thing to do, and it is a kind of anti-comedy. It is dark and subversive. It is a play which seems to me to be primarily about law and justice, but it sees the law as essentially an oppressive exercise of power and not having much to do with justice. Even the end, in which things work out alright, is deeply ambiguous, and it is not clear after a little thought if justice has been done. The play ends with 3 or 4 marriages, the critical relationship with the Duke and Isabella, effectively the two main characters, is left ambiguous and in my opinion should not be resolved in a staging. In fact the whole play is ambiguous. Shakespeare takes an urban myth, shakes it up, turns it into a comedy and then pretends to resolve it, without actually resolving anything.
The character of Isabella, who is the sister who suffers the sexual blackmail, is about to start a life as a nun. The modern reader, will be inclined to miss the fact that this does not mean that she is unattached, but instead that she is engaged to God, and so the person on whom she would be cheating if she were to give into the blackmail would be God himself. The fact that she may or may not accept the Duke's proposal at the end, means that she may not stay true to her original suitor, God, and this would not have been lost on an Elizabethan audience.
The whole play perversely shoehorns this uncomedic material and floats the figure of the Duke over the whole thing. It is the Duke's own laws that bring about the situation, but he is not the one enforcing them. That is a lackey who is notorious for his uprightness, becomes aroused by Isabella's moral rectitude and blackmails her into sex to his own horror at his own hypocrisy. He is essentially sentenced for this crime into marrying an ex-girlfriend. The Duke meanwhile is an immensely complex character. He moves the plot as a deus ex machina and is a mixture of vanity, eccentricity, wisdom and Machiavellian scheming. He is a deeply ambiguous figure, perhaps the most ambiguous that Shakespeare wrote.
The play is also stylistically strange. It is written in verse for the first half and then suddenly breaks midscene into prose. I find it impossible to believe that this is anything other than on purpose and that it does not represent Shakespeare trying to hit a deadline, but what it means and why it was done, I have no idea.
All of this complexity and genius on display, might make one think that Measure for Measure would be one of my favorite Shakespearean plays. However, there is something about it that does not quite gell. Apparantly, it is only really since the Second World War that Measure for Measure has become popular, perhaps because it is about sex and we live in a more permissive time. However, I think that earlier generations were right, and it not amongst Shakespeare's best. That having been said the work is brilliant and would be amongst the best of virtually any other author.