Here is a plot outline that you can alter to the diagram that you need:
1. Exposition - The beat policeman moves impressively and has a "watchful eye" as he moves up and down his street. He slows his walk at an open doorway and approaches the man standing there.
2. Rising Action - The man in the doorway lights a cigar and talks with the officer, telling him that he is meeting an old friend after twenty years. The policeman converses with the man, asking about his friend. The man assures the policeman that his friend will come; the policeman asks him how long he will wait for his friend. "I'll give him half an hour, at least," replies the scar-faced man with a diamond tie pin.
3. Climax The friend calls out "Bob," and the man in the doorway greets "Jimmy Wells"; they walk down a street until they stand under a bright street light. Bob realizes that "Jimmy" is not his friend. An undercover policeman tells 'Silky Bob,' "You've been under arrest for ten minutes."
4. Falling Action - The plainclothesman gives 'Silky Bob' a note, telling him it is from Patrolman Wells.
5. Denouement (Resolution) 'Silky Bob' reads the note from his old friend who writes that when he recognized him as a wanted man, he had a plainsclothesman come by and pick him up.
I believe you are referring to the story "After 20 Years" by O Henry. This is a short story about a man who is waiting for a friend who he has not seen in twenty years. As he waits, he is approached by a police man, and he explains the situation. The police man leaves, and the friend the man is waiting for shows up. The two begin to catch up and the supposed "friend" announces that the man is under arrest. It turns out the police man was the friend, but when he saw his old friend waiting, he recognized him from a wanted poster and got a plain clothes police man to arrest him.
I am going to move this thread to the O. Henry group.
The policeman is walking down the street checking doors. He see's the man standing in the dim doorway smoking a cigarette. He stops to talk to the man, and he is told about the childhood friends who agreed to meet at this very spot after 20 years. They talk about how the area has changed. He realizes this is the criminal that Chicago has said might be headed their way, but he leaves. When the man who is supposed to meet his friend there shows up, they talk about how each of them has changed. The criminal is arrested, and he says that he new 20 years couldn't have changed a man's features so much, and the undercover cop says no but it can change a good man into a criminal.