O’Henry builds the suspense in this story by introducing the two characters, and then not letting you know who is who right away. They are two old friends who are meeting after twenty years, and he slowly feeds you details of their backstory. It later is revealed that one is a police officer and one is a crook, and the twist is that the police officer has been talking to the crook the entire time—and he didn’t know it! Neither does the reader, until the end.
In the beginning, we just see a very determined policeman seemingly walking his beat. His impressiveness is habitual. This means that he just can’t help it, because it is part of who he is by now. Duty and responsibility are ingrained in him. O’Henry is building suspense already, because that will be important later.
Then, in the doorway of a darkened hardware store, he sees a man with an unlit cigar. The conversation gets interesting. First of all, the man, “spoke quickly.”
"It's all right, officer," he said, reassuringly. "I'm just waiting for a friend. It's an appointment made twenty years ago. Sounds a little funny to you, doesn't it?...”
There are many telling things about this encounter. A darkened doorway? An unlit cigar? The man’s quick reaction to the police officer? Warning bells should be going off in the reader’s mind—this is foreshadowing.
The mean tells a story about making an appointment to meet twenty years ago with a friend named Jimmy. He lights his cigar and his face—and a scar—are clearly shown.
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.
The interesting thing about this is that the cop does not react, other than to say it is an interesting story and ask if he has heard from his friend. Bob, the cigar smoker, comments that Jimmy will be there because “he always was the truest, stanchest old chap in the world.”
The cop leaves and another man, comes, pretending to be Jimmy. Bob says immediately that he has changed a lot. That is another signal to the reader that it may not be Jimmy after all. It builds suspense, because we do may not know what is going on, but we know something is not right.
"You're not Jimmy Wells," he snapped. "Twenty years is a long time, but not long enough to change a man's nose from a Roman to a pug."
After that, the cop tells him he has been under arrest for ten minutes! Then he gives him a note from the real Jimmy, who tells him that when he met him he knew he had to arrest him but did not have the heart to do it himself, out of friendship.
The twist in this story is that Jimmy did show up, all along, and he both did and did not arrest Bob. Ironically, Bob turned into a criminal and Jimmy into a cop. Jimmy wanted to do the right thing and the wrong thing at the same time, and found a way to do his duty and do right by his friend. O’Henry gives us little hints all along, leading up to the final twist. The unlit cigar and then the flash that showed Jimmy who Bob was and allowed him to recognize his friend as a wanted man, and the fact that Jimmy never acknowledged who he was, as well as Bob’s reaction to seeing a cop, all foreshadow the surprise ending.
Twenty years is a long time to keep a promise. As you can see, it really did not change the kind of person Jimmy was. He remained the loyal and honest, and dependable person Bob said he would be. Bob said he would be there, and he was. Bob said he could count on Jimmy, and he could.