The man standing in the doorway seems out of place because it is a hardware store that is closed and the lights are out.
When Jimmy the cop is walking down the street, he does so with a habitual awareness of his surroundings. During his survey of the neighborhood, he notices a man who seems out of place.
When about midway of a certain block the policeman suddenly slowed his walk. In the doorway of a darkened hardware store a man leaned, with an unlighted cigar in his mouth. As the policeman walked up to him the man spoke up quickly.
The man is clearly familiar with policeman, because he is aware that he is noticed. He tells Jimmy that it’s all right, because he is waiting for someone. He explains why he is there, in a hardware store, late at night. You would not expect someone to be waiting in the doorway of a hardware store, where the lights are off, when the store is closed. This is why he is so quick to explain himself to the cop.
He tells him he made the appointment twenty years ago, when the place he was standing was not a hardware store.
“…Sounds a little funny to you, doesn't it? Well, I'll explain if you'd like to make certain it's all straight. About that long ago there used to be a restaurant where this store stands--'Big Joe' Brady's restaurant."
Now it makes perfect sense to the reader why the man would be standing in the darkened doorway of a hardware store late at night. He made the appointment long ago. At that time, it was a diner and not a hardware store. It would have been open at this time of night.
We also find out that the man he was supposed to be meeting is the cop he is talking to, but Bob (that’s the man in the doorway) does not know that yet. Jimmy is a good man. Most cops probably are.
When Bob lights a cigar, Jimmy sees his face. He realizes that Bob is a criminal. He never has let on who he is, so he keeps going. He gets another cop to pretend to be him (Jimmy), and they talk until the cop arrests Bob and gives him a note explaining everything. Still, both men kept their appointments!
This story is about friendship, and irony. It is also about people staying true to themselves. Ironically, both Jimmy and Bob remained true to themselves. They both kept the appointment. It was because of their personalities that they kept it, but it was also because of those personalities that Jimmy because a cop and Bob became a criminal.