It is the dialogues of "After Twenty Years" that propel the plot and reveal O. Henry's signature irony in this story.
From these dialogues, then the main characters meet and speak in the exposition, the problem of the plot is presented as they converse, the climax occurs as Bob speaks to the plain clothes policeman, and the resolution comes with the written note from Officer Wells as a communication with "Silky" Bob.
The stranger who stands in the doorway of what once was a restaurant divulges to the policeman on his beat that he is waiting for an old friend. It was twenty years ago, he says, that they parted ways, and they agreed on that night to meet again. He adds with dramatic irony,
"We figured that in twenty years each of us ought to have our destiny worked out and our fortunes made, whatever they were going to be."
Little does this stranger know that he is talking to his old friend Jimmy, who is now the policeman in his old neighborhood. And, little does the policeman realize until the stranger strikes a match and a scar near his right eyebrow is revealed as well as a large diamond scarfpin that this stranger is "Silky" Bob, who is wanted in Chicago.
But, Officer Jimmy Wells displays no recognition; instead, he continues to talk to the friend of his youth, asking him if he has done well in the West. Bob answers, "You bet!" adding that he hopes Jimmy has also fared well. "He was a kind of plodder," he comments. "It takes the West to put a razor-edge on him [a man]." These words indicate to Jimmy that Bob has the criminal mentality, so he says to Bob, "Hope your friend comes around all right. Gong to call time on him sharp?" Bob replies that he will give his friend at least half an hour leeway; so, the policeman knows he has time to return to the precinct.
There, Jimmy writes to his former friend, who has turned criminal, and asks another policeman to deliver this note. With his collar pulled up, this plain clothes officer meets Bob in the doorway. However, as they walk, Bob sees the man's features under the street light. "You're not Jimmy Wells!" The officer then informs Bob he is under arrest, and hands him Jimmy's note. In this note, Officer Wells informs him of his being the policeman as he writes that he recognized him by the light of his match as the man wanted in Chicago; he also tells Bob that he did not have the heart to arrest him, so he "got a plain clothes man to do the job."