The Duke of Vienna decides to leave the city for a while, appointing his deputy, Angelo, to govern in his absence. Angelo resolves to revive vigorous laws against sexual licence, which have fallen into disuse. Under these laws a young gentleman, Claudio, is sentenced to death for having made his fiancee Juliet pregnant before their wedding ceremony. Claudio's sister Isabella, a novice nun, is persuaded by his friend Lucio to plead with Angelo for her brother's life. Angelo promises to free Claudio if Isabella will sleep with him. She refuses and tells her brother that she must keep her honour and he, therefore, must die. The duke has stayed in Vienna disguised as a friar observing the new regime and he now persuades Isabella to pretend to accede to Angelo's demands. Her place in Angelo's bed is secretly taken by Mariana, Angelo's former fiancee whom he deserted. After the event, Angelo betrays Isabella by trying to carry out Claudio's execution but his plans are foiled by the duke's intervention.
Duke would appear to be in complete control of his situation, but powers outside the scope of the play force him into disguise. The Duke leaves a sex crazed Puritan in charge of the state, but remains in the city in disguise. He complicates matters in scene after scene. Among other things, the Duke throws in the bed trick (a la Anne Cecil), and saves another character from execution by the skin of his teeth (Southampton, following the Essex episode).
In the play's first scene, the Duke pretends to leave Vienna, but dons a disguise. By way of explanation, he explains:
DUKE: I love the people
But I do not like to stage me to their eyes.
Though it do well, I do not relish well
Their loud applause and aves vehement,
Nor do I think the man of safe discretion
That does affect it.
Talking through the Duke, de Vere talks about his reasons for writing anonymously, and the prospect that a man of no small ego may not be remembered in history.
The Duke sets Mariana - a chaste and wronged wife - upon the state's unsuspecting deputy (Angelo). Angelo has sex with his long-ago betrothed Mariana, while thinking he is sleeping with another woman (Isabella). Isabella becomes the play's advocate for truth telling and mask removing. The Duke dismisses the pleadings:
ISABELLA: It is not truer he is Angelo
Than this is all as true as it is strange.
Nay, it is ten times true. For truth is truth
To th'end of reckoning.
DUKE: Away with her! Poor soul,
She speaks this in the infirmity of sense!
This is a retelling of one of the explanations for the birth of Anne Cecil's first child Elizabeth.
The expression "truth is truth" is a component of the de Vere family motto. In a letter to Willliam Cecil, de Vere used words similar to those of Isabella: "Truth is truth, though never so old, and time cannot make false that which was once true."
By the late 1590s, Anthony Munday - de Vere's longtime secretary - published a translation of Sylvain's Orator under the name Lazarus Piot. In the biblical story, Lazarus is lame and sickly (like de Vere at the time) and piot is slang for saucy chatterbox. In one of the Orator's tales, a ravished maid demands that the rapist be made her husband, and that he be sentenced to death. This same plot is incorporated into the play.