The humorous tone of the story is most effective when a more serious tone would ordinarily be used. A prime example of this comes when the two hapless kidnappers receive a reply to their ransom note from the boy's father. Instead of expressing fear about his son's safety, as would normally be the case, or making arrangements to pay the ransom, Mr. Dorset turns the tables on Bill and Sam, actually demanding that they pay him—the princely sum of $250, no less—and return little Johnny so he can take them off their hands. As if we didn't already know it, Johnny's a bit of a handful, so we can understand why his father wants to receive some kind of compensation for being reacquainted with his mischievous, annoying brat of a son.

In the second paragraph of O. Henry's story, the narrator establishes a humorous tone with his description of the town in Alabama where he and Bill Driscoll got the idea for the kidnapping:

"There was a town down there, as flat as a flannel-cake, and called Summit, of course."

 

"Flannel-cake" was a colloquialism for pancake, so it is a type of ironic humor for a town whose geography was flat to be named "Summit," a term for the highest point of a mountain. The addition of "of course" intensifies the humorous tone; it is absurd for a town so flat to be so comically misnamed.  This opening is an appropriate set-up, tonally, for a story of a criminal plan that will, likewise, go absurdly, comically wrong when the father of the kidnapped boy demands money from the kidnappers to take him off their hands.