There is a slight weakness in the ending of O. Henry's story. The reader is expected to admire Jimmy Wells for doing his duty in having "Silky" Bob arrested. This makes Jimmy seem like a "staunch" character (to borrow from Bob's description of him). However, he was not "staunch" enough to make the arrest himself. This may be a sign of compassion, but it is also a sign of fecklessness. What would his superiors think of him if they found out that he had been talking face to face with a man who was wanted by the law and didn't make the arrest himself? Could he explain that dereliction of duty to them as well as he explains it to Bob in his note?
After spending some minutes talking to Bob without being recognized, Jimmy takes pains to establish how long his old friend will be waiting there in the doorway.
"I'll be on my way. Hope your friend comes around all right. Going to call time on him sharp?"
"I should say not!" said the other. "I'll give him half an hour at least. If Jimmy is alive on earth he'll be here by that time. So long, officer."
"Good-night, sir," said the policeman, passing on along his beat, trying doors as he went.
Jimmy makes sure that Bob will be waiting there long enough for him to get to the station and enlist a plain clothes detective to make the arrest. It should be noted that both at the beginning and end of the encounter between the two old friends, O. Henry specifies that the uniformed cop is patrolling his own beat, which just happens to include the hardware store where Bob is standing. If this were not Jimmy's regular beat and Bob did not see him trying doors along the way as he approached and departed, then Bob might become suspicious.
Bob is smart. Bob is on the lam. Bob is not an easy man to fool. If he had not seen Jimmy in the process of patrolling his beat, he might have asked himself some questions, such as: "Why did this uniformed cop suddenly appear out of nowhere at just the time that I have an appointment to meet Jimmy Wells?" "And why did the uniformed cop just walk off after talking to me for a few minutes?" "Where did he come from?" "Where was he going?" "And why???"
If Bob hadn't been suspicious while he was talking to the cop he didn't recognize as his old friend, he could easily have become suspicious after that cop walked away. He might have suspected that he was being set up. In fact, he might have suspected that Jimmy himself was the one who was setting him up. Because Jimmy is the only person in the whole wide world who knew that Bob would be at that precise location at ten o'clock on that particular date. Such things happen all the time--and they had probably happened to Bob before in his career.
The reader's own possible suspicions are allayed by the apparent fact that the uniformed cop is obviously patrolling his beat. That is what enables O. Henry to deliver his biggest surprise at the end of his story. The cop who had been chatting with Bob at the beginning was actually Bob's old friend Jimmy Wells, now wearing a policeman's uniform. It takes a very good writer to introduce a character without really introducing him.
The premise of "After Twenty Years" by O. Henry is a simple one: two young friends part ways, one stays in New York and one heads West, and agree to meet at a specific time and place twenty years later. What happens at that time and place is the crux of the story.
Bob has been out West, and he has obviously had some success. He is dressed well and has a diamond stickpin and watch. He is waiting on the corner where he and his friend Jimmy had agreed to meet, and a police officer approaches him. Concerned that the officer might think he was up to no good (which is quite ironic, given what we learn later), Bob lights a cigarette (by the light of which the officer can see the man's face more clearly) and tells him the story of why he is here.
"Twenty years ago to-night," said the man, "I dined here at 'Big Joe' Brady's with Jimmy Wells, my best chum, and the finest chap in the world. He and I were raised here in New York, just like two brothers, together. I was eighteen and Jimmy was twenty. The next morning I was to start for the West to make my fortune. You couldn't have dragged Jimmy out of New York; he thought it was the only place on earth. Well, we agreed that night that we would meet here again exactly twenty years from that date and time, no matter what our conditions might be or from what distance we might have to come. 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."
Bob is correct; each of them has found his destiny. Bob, as it turns out, has become an infamous Chicago gangster, and Jimmy has become an upstanding policeman. While Jimmy recognizes Bob, Bob does not see the officer well enough to recognize him as his old friend Jimmy.
Now Jimmy has, it seems to me, three choices: does he arrest his childhood friend (who is now a wanted criminal), does he just walk away and pretend he never saw Bob, or does he acknowledge Bob but ignore the fact that Bob is a wanted criminal. None of the choices are good ones for Jimmy.
Clearly the twenty years that have elapsed have, as Bob said, determined his destiny. He went West and chose a life of crime. Though he is still a loyal friend to Jimmy because he showed up here tonight and is even willing to wait an extra half hour for him, Bob is not a good man or a good citizen. His choices have made him rich, but they certainly have not made him a better man.
Those same twenty years have shaped Jimmy's destiny, as well. Jimmy chose to spend his life upholding the law and, ironically, protecting others from people just like his old friend Bob. This is still not an easy choice for Jimmy, for he is also a loyal friend. Even so, Bob unknowingly reveals what Jimmy must do when he describes him this way:
"But I know Jimmy will meet me here if he's alive, for he always was the truest, staunchest old chap in the world."
So now we realize that Jimmy has no choice; because he is a trustworthy and staunch (loyal, committed, and strong) police officer, he has to arrest Bob. He manages to do the right thing while still remaining a friend to Bob, sending someone else in to make the arrest. That is the best he could offer his old friend; he spares Bob the embarrassment and indignity of having to face his old friend in handcuffs.
If I had been Jimmy, I would hope that I would have been both compassionate and moral enough to do exactly the same thing.