The Duke makes a grand entrance at the city gate, where a big crowd is there to greet him, including Angelo and Escalus. (Hmm. So much for the Duke's earlier claim that he doesn't like to make a spectacle of himself.)
The Duke turns to Angelo and Escalus and says something like "I hear you two have been doing a bang up job in my absence." Duke Vincentio proceeds to make a big deal about how grateful he is for Angelo's service to Vienna.
Friar Peter and Isabella arrive and the Duke pretends not to know who they are.
Isabella accuses Angelo of blackmailing her into sleeping with him.
Angelo accuses her of being crazy and blames her behavior on her brother's recent death.
Isabella calls Angelo a "hypocrite" and a "virgin-violator."
The Duke orders Isabella be carted off because it's obvious the poor girl has gone mad.
Then the Duke pretends to change his mind and says that, for a supposed crazy lady, Isabella sure does seem like she's capable of reason.
Lucio interrupts and the Duke tells him to pipe down – nobody gave him permission to speak.
Isabella proceeds to tell a partial lie. She says she slept with Angelo (she didn't) to save her brother's life and that Angelo reneged on their deal, sentencing Claudio to death.
Duke Vincentio pretends to be horrified and declares that Angelo would never, ever, ever put a man to death for doing something that he himself is guilty of – if he did, that would make Angelo a big, fat hypocrite.
The Duke orders Isabella to prison for lying.
Isabella says she wishes Father Lodowick was here now – he'd clear up this whole mess. (Aha! We now know that the Duke has been calling himself "Friar Lodowick"!)
Lucio chimes in that he knows the guy and has also heard Friar Lodowick bad-mouthing the Duke.
The Duke demands to see "Friar Lodowick," but Friar Peter says "sorry, he's sick and can't make it."
Isabella is carried off to the slammer.
Mariana, who is wearing a big, black veil over her face, enters and acts all mysterious, claiming to be neither a married woman or a "maid" (an unmarried virgin). She also claims that her husband "knows not that ever he knew" her. (That's a cryptic way to say that Angelo doesn't know that he slept with her. Mariana is playing on the biblical idea that sex is "carnal knowledge.")
Lucio says that her husband must have "been drunk" if he didn't know he slept with his wife.
Mariana announces that Isabella is lying about sleeping with Angelo. She knows this because when Angelo was supposedly "fornicating" with Isabella, he was in Mariana's arms.
Mariana declares that, even though Angelo thinks he knows that he was with Isabella, he doesn't know that he was getting to know her (Mariana's) body.
Mariana removes her veil and yells at Angelo for breaking their marriage contract. She also reveals the clever bed trick.
Angelo admits that he was once engaged to Mariana but, when her dowry was lost at sea, he broke it off.
(Note: Mariana's story is different. She says they took vows in a "hand-fasting" ceremony, which, according to common law is a legally binding marriage because they consummated their marriage last Tuesday night in the secret garden.)
Angelo says Isabella and Mariana are in on a secret scheme to ruin his life and the Duke pretends to believe him.
The Duke leaves the Provost to sort out the mess and then returns a few moments later disguised as Friar Lodowick.
Escalus accuses the friar of convincing Isabella and Mariana to slander Angelo.
Lucio steps up and claims that Friar Lodowick has also been talking trash about the Duke.
Lucio yanks off the Duke's hood and the Duke is all "Aha!"
Angelo realizes that the Duke knows all about his recent shenanigans and immediately 'fesses up to everything.
The Duke orders Angelo to marry Mariana in a religious ceremony, which will most definitely make the marriage legal. They leave the stage to get hitched.
He then turns to Isabella and says, "I bet you're wondering why I didn't save your brother's life – but Claudio is in a better place now."
Angelo and Mariana return to the stage, man and wife.
Duke Vincentio says that Angelo must be put to death. The only way justice can prevail is if Angelo pays for his crime against Claudio with his own life: "An Angelo for a Claudio, death for death [...] Measure still for Measure."
Mariana begs Isabella to help her save Angelo.
Isabella kneels down before the Duke and begs for Angelo's life. She says that Claudio was justly punished for his crime of fornication but Angelo shouldn't be put to death because, even though he thought he was committing a crime and a sin by sleeping with her, he didn't actually do anything wrong because he was tricked into sleeping with Mariana.
(We know. This doesn't make any sense. The problem with Isabella's argument is that Angelo has slept with Mariana, which is exactly what Claudio's did with Juliet. In other words, Angelo and Claudio have committed the same crime.)
The Duke pardons Angelo and then turns to the Provost and pretend-fires him for executing Angelo.
The Provost disappears and returns with... wait for it... Claudio, who hasn't been beheaded after all!
The Duke, who is really pleased with himself, turns to Isabella and says he's got one more awesome surprise for her – he wants to marry her, lucky girl that she is.
Isabella is completely silent. (Is it because she's overjoyed or because she's horrified? You decide.)
Then the Duke turns to Lucio and says he'll be whipped and then hanged. First, though, the Provost will make a public announcement that, if any woman comes forward and claims that Lucio is her baby daddy, then Lucio must marry her. (Remember, Lucio confessed earlier to getting a woman pregnant and then lying about it in court.)
Lucio begs the Duke not to force him to marry a prostitute and says he'd rather be tortured and hanged.
The Duke changes his mind about hanging Lucio and decides that marriage is a just punishment.
Duke Vincentio makes a big speech about how wonderful it is that four couples are either engaged or already married.