The Duke's statement that Angelo's "desert speaks loud," and should have "a forted residence 'gainst the tooth of time" is especially ironic; for Angelo's desert is non-existent, and has already been corrupted, not by time but by lust. The Duke plays his part, acting like he is ignorant of the recent goings-on in the town; Isabella plays the part of a wronged woman with great dramatic skill.
All that has been pre-arranged and staged by the Duke is being executed before his eyes; his manipulations are being acted out in a public place, to the end results that the Duke wantsjustice, and Angelo's exposure. reality to the assembled party; she makes the good point that anyone can appear to be just, but can be completely corrupt within, as Angelo is.
The Duke is still coming off a little like Prospero, making himself seem a benevolent advocate for Isabella, and one who used his power rightly, even when he was in disguise.
But, though the Duke says he did "not [change] heart with habit," he did abuse his disguise as a priest to get people to trust him and believe in his unproven honor.
Friar Peter has Mariana come forward, as the Duke urges Angelo to be judge of his own case.
Mariana says she will not show her face until her husband bids her to do so; she says she is not a maid, a widow, or a wife, and then tells them that she has slept with her husband, whom Isabella is accusing.Isabella is allowed to speak her tale, and tells of how she went to Angelo to plead for her brother's life, and had to sleep with Angelo to get him released, although he was executed anyway.The Duke, of course, along with Friar Peter, make a show of not believing her, and say that Angelo is too good by reputation for these allegations to have any weight with them.Immediately after his appearance, the Duke proposes marriage to Isabella, perhaps using her flood of happiness at seeing her brother to secure her quick consent.Lucio is then sentenced to marrying the prostitute he got pregnant, as punishment for slandering the Duke.One ironic note, though, is that Angelo senses that the two women are "but instruments of some more mightier member"; in fact, they are playing the parts the Duke has set out for them, and this will soon be revealed to all.Angelo's repentance seems a bit abrupt, especially his statement that he deserves to die for what he has done.Lucio gets angry at him for accusing him of saying bad things about the Duke, which in turn Lucio accuses the friar of; then, Lucio pulls off the ' Friar's' disguise and finds that it really is the Duke after all.Angelo then knows that he is undone; he says he will confess what he has done, and forego a trial of his wrongdoing.He does this for no reason other to have her be very happy, and perhaps look even more just and heroic, when it is revealed that he actually saved Claudio's life.The Duke did his duty as a ruler, seeing to it that justice was kept and no undue sentence performed; however, through this public performance of accusation, repentance, and grief, he shows the extent of his power, and makes himself look even more just, important, and beneficent since he appears to have single-handedly acquired justice for everyone.