Most good stories start with a fundamental list of ingredients: the initial situation, conflict, complication, climax, suspense, denouement, and conclusion. Great writers sometimes shake up the recipe and add some spice.
Initial Situation
Vienna's Looking a lot like Sin City.
For the past fourteen years, the people of Vienna have been running amok – disobeying laws, fornicating, and visiting the city's brothels. STDs and illegitimate children are rampant. Duke Vincentio thinks enough is enough and puts a strict deputy in charge of enforcing Vienna's laws while the Duke is "away" on business.
Conflict
Claudio is busted for "fornicating."
With Angelo in charge, Claudio is sentenced to death for "fornicating" with his girlfriend and getting her pregnant. He's headed for the chopping block if someone doesn't intervene on his behalf...
Complication
Isabella leaves the convent to save her brother.
Claudio's sis, Isabella, is about to become a nun, but when she hears her brother is in the slammer, she pleads with Angelo to have mercy.
Climax
Angelo propositions Isabella.
Angelo gets all hot and bothered by Isabella's virtue and says he'll set Claudio free if Isabella sleeps with him. Isabella immediately refuses (saying she'd rather die) and Angelo gives her a day to think about it.
Suspense
The Duke hatches a plan.
Meanwhile, the Duke hasn't left Vienna – he's been running around Vienna disguised as a friar. When he finds out about Angelo's hypocritical behavior, he comes up with a plan. Isabella should agree to hook up with Angelo but she will send Angelo's jilted, ex-fiancé (Mariana) to sleep with Angelo in her place. (It's going to be dark so Angelo will never know the difference.) Angelo will be forced to marry Mariana, Isabella will get to remain a virgin, and Claudio will be set free.
Denouement
Bed trick!
Angelo thinks he's steamed up the windows with Isabella but has, in fact, slept with Mariana (the girl he dumped when he found out she didn't have a dowry). But, Angelo reneges on the deal and orders Claudio to be put to death anyway.
Conclusion
Everybody's getting hitched.
The Duke finally reveals his identity and patches things up. Claudio is set free (which means he can now marry his girl, Juliet), Angelo is forced to marry Marianna, and the Duke himself proposes to Isabella (who may or may not be excited about the prospect of marriage).