O. Henry's touching short story with the mildly surprising ending is about two old friends, twenty years, and life's vagaries and changes. Bob and Jimmy Wells at the ages of eighteen and twenty were set to go off on their separate ways to make their fortunes and find their destinies. They agree to meet back together in twenty years from the date and time of their parting to see each other again. A survey of the movement of the story will help sort out what Bob is.
Both are New York boys, Bob has an ambition to meet his destiny and make his fortune in the West while Jimmy desires to remain firmly rooted in New York City. As Bob tells the city policeman patrolling the street where Bob and Jimmy are to meet, Bob moved around the West quite a bit and the two friends eventually quit corresponding over the years:
the West is a pretty big proposition, and I kept hustling around over it pretty lively.
Bob also says he knows that if Jimmy is still alive, he will show up at the appointed place and confirms that he will wait if Jimmy is late in coming. When a tall man huddled in an overcoat approaches, he and Bob are reunited and start to walk "around to a place" for a meal.
In the light of a drug store window, Bob sees that the man isn't really Jimmy. Here is where we discover--if we weren't suspicious of it already because of all the mentions of the West and all the diamonds sparkling on Bob and his possessions--that Bob is a criminal from Chicago who is wanted by the Chicago police. Jimmy turns out to be an impostor come to arrest him.
The association of great wealth, criminal activity, historical time period (c. 1904), and the city of Chicago lead the reader (especially in O. Henry's day because it was a contemporaneous time period) to understand that Bob is part of the Chicago gangster scene, reminiscent of the slightly later Al Capone. Jimmy Wells, of course, was the patrol officer with whom Bob spoke, as his note explains:
"Bob: I was at the appointed place on time. When you struck the match to light your cigar I saw it was the face of the man wanted in Chicago. Somehow I couldn't do it [arrest you] myself, so I went around and got a plain clothes [police] man to do the job. JIMMY.”