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)
The best laid plans . . .
This section starts out with the basic notion: hey, let's kidnap a kid for some quick cash! It establishes the arc of the story, introduces us to the major characters, and even gives us a shoe for the rest of the story to drop. After carefully choosing a small town with an ideally unmotivated populace, Sam and Bill locate what they think is a perfect victim. The key word there being "think."
Rising Action (Conflict, Complication)
Would you like a bag of candy?
The complication here is simple and to the point. The "easy" kidnapping turns into a nightmare as Johnny proceeds to torment Sam and Bill (mostly Bill) with a lively game of Indian Chief vs. Bumbling White Men. Suddenly, a well-laid plan goes horribly wrong, and our heroes are left to make the best of it until they can finally unload the kid and get their money. The sooner the better…
Climax (Crisis, Turning Point)
I readily take you at your word.
And then the twist hits. Not only is Johnny's father unwilling to pay to get him back, he demands that Sam and Bill pay him before accepting Johnny. At this point, Sam and Bill are so demoralized that they're happy to pay whatever they have just to get the little hellion out of their hair for good.
Falling Action
"What's two hundred and fifty dollars, after all?"
The falling action moves pretty swiftly as Sam and Bill pay Ebenezer and deposit Johnny off in the middle of the night. It was probably better not to alert the neighbors that he's coming back into their little neck of the woods. With the twist out of the way, this section exists mainly to guide us to the closure, along with a little extra drama to determine if Ebenezer's going to be true to his word.
Resolution (Denouement)
"It was either that or Bill Driscoll to the madhouse."
There are a surprising number of comedies—cartoons mostly—that end with some miscreant running for his life. O. Henry gives us a shining example of that kind of ending, peppered with a subtle irony for which he became known. Sam and Bill hit upon this plan because they didn't want to get run out of town. And it turns out, they get run out of town. Sure, there's no torch bearing mob behind them, but after everything they've put up with, a torch-bearing mob would probably be preferable. Careful what you wish for boys…