What is the plot diagram for "After Twenty Years"?

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.

http://www.enotes.com/o-henry-salem/o-henry

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.

What is the climax in "After Twenty Years" by O Henry?

The climax in a story is the peak of emotional intensity. In "After Twenty Years" this peak is reached when Bob and the arresting officver arrive at the corner drug store which is brilliantly illuminated. Here Bob realizes that his companion can't be his old friend Jimmy Wells. He is shocked to realize he has been confessing his sins to a complete stranger. As O. Henry puts it:

The man from the West, his egotism enlarged by success, was beginning to outline the history of his career.

Then the actual climax is reached when the stranger tells him:

"You've been under arrest for ten minutes, 'Silky' Bob."

The plain clothes detective who makes the arrest hands Bob a note from his friend Jimmy. This contains some necessary information, but it is not the climax. It is more of an explanation of much of the entire story--something like a summation. Bob realizes that the policeman he had been talking to at the beginning of the story was actually his old friend Jimmy Wells and that Jimmy had remembered their appointment and had kept it. He also realizes that Jimmy had changed a great deal in the past twenty years, just as Bob had changed a great deal himself. The friendship between these two men, which had been so strong twenty years before, was only a ghost of what it had been. That is perhaps the point of the story: people change. Nothing lasts. Jimmy still cared enough about Bob to feel he could not arrest him personally, but his sense of duty was stronger than his sense of friendship. His note to Bob reads, not only like an apology, but like a final goodbye, a statement that they can no longer be friends because of the separate paths they have taken in life.

O. Henry never describes what sorts of crimes 'Silky' Bob has committed. The closest the author comes to acknowledging that Bob is actually guilty of serious crimes is in the sentence:

The man from the West, his egotism enlarged by success, was beginning to outline the history of his career.

Note that he was only "beginning." It was probably lucky for him that they reached the lighted drugstore windows as soon as they did. Otherwise he might have confessed too much. But O. Henry seems to have wanted to soften the magnitude of Jimmy's betrayal. The arresting officer only tells Bob:

"Chicago thinks you may have dropped over our way and wires us she wants to have a chat with you."

It may be that Bob will get off with just a long and serious interrogation, and perhaps a stern warning. "A chat" doesn't sound so bad. Jimmy's note only says that Bob looks like the "man wanted in Chicago." At this point Bob is only wanted for questioning. They don't call him 'Silky' Bob for nothing. The reader knows he is in trouble but suspects that he may be able to talk his way out of whatever jam he is in. No doubt O. Henry gives him the nickname 'Silky' Bob to suggest that he is not a violent criminal but a con artist.

O. Henry spent some years in prison for embezzlement. He learned a lot about crime and criminals from his life experiences in prison and afterwards when he got out and spent some time with criminal associates. His stories such as "A Retrieved Reformation," "The Cop and the Anthem," and "The Ransom of Red Chief" show his mixed feelings about the denizens of the underworld. In "After Twenty Years," the reader is not glad to see Bob arrested but feels sorry for him because he was betrayed by an old friend whom he had traveled a thousand miles to meet for dinner and for old time's sake.