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.
Exposition (Initial Situation)
Lock Up Your Daughters
We meet the main characters and learn that Polly Peachum has disappeared with a mysterious stranger known as Mac the Knife. This sets up the main conflict between her dad, Mr. Peachum, and her new husband, Macheath.
Rising Action (Conflict, Complication)
A Lovely Wedding
Polly and Mac marry in a secret ceremony, and Peachum decides that he'll get Mac thrown into jail for all of his unpunished crimes. Things are rough for Mac, who has to go into hiding in order to avoid prison.
Climax (Crisis, Turning Point)
In the Slammer
Mac ends up in jail, escapes, then is betrayed once again and sent back to be executed. He's headed to the gallows after all of his friends have forsaken him, and it looks like, for once, there's no way out for Mac the Knife.
Falling Action
Off With His Head
At the last minute, a horseman comes to pardon Mac and set him up with a really cushy life. It's totally unexpected and changes everything the play seems to be leading up to. Rather than see an execution, the audience is jarred by the silliness of the turn of events.
Resolution (Denouement)
All's Well That Ends Well?
The characters moan a bit about how in real life things don't work out so nicely, but they're lucky to be in an opera instead of in real life. Mac is rich and everyone else is still poor and complaining about it.