What is the summary of "After Twenty Years" by O.Henry?

O. Henry critic Boris Ejxenbaum wrote that the O. Henry
short story “gravitates toward maximal unexpectedness of a finale concentrating around
itself all that has preceded.”  This method is certainly evident in "After Twenty Years"
as a policeman walks his beat and happens upon a man standing in an empty
doorway.

In order to light his cigar, the man strikes a
match, revealing a square-jawed face with a think scar on the eyebrow.  He assures the
policeman that he means no harm; he is simply waiting for an old friend whom he promised
he would meet at 'Big Joe Brady's' restaurant after twenty years have passed.  He and
Jimmy Wells, his best "chum" agreed to meet no matter what the conditions might be or
from what distance they had to come.  When the policeman queries if the friend has heard
from Jimmy, the man replies that he did for a while, but after he went West they lost
contact with one another.  However, Jimmy is the "truest, staunchest, old chap in the
world.  He'll never forget."

The man waiting looks at a
diamond studded watch.  "Three minutes to ten.....It was exactly ten o'clock when we
parted."  The policeman asks if he will "call time on him sharp."  No, replies the
waiting man. "I'll give him a half an hour at least.  If Jimmy is alive on earth, he'll
be here by that time."  The policeman bids him
goodbye.

After nearly twenty minutes a man with an overcoat
collar turned up walks directly to the waiting man, asking "Is that you Bob?"  And the
waiting man returns, "Is that you Jimmy Wells?" Commiserating upon what a long time
twenty years has been, the waiting man remarks that Jimmy is taller than he remembers. 
As they pass a drugstore on a corner brillant with electric lamps, Bob stops suddenly,
realizing  the man with him is not Jimmy Wells:

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"Twenty years is a long time, but not long
enough to change a man's nose from a roman to a pug."

"It
sometimes changes a good man into a bad one," replies the tall man, who continues,
"You've been under arrest for ten minutes 'Silky
Bob.'"

Before the tall man
escorts Silky Bob to the police station, he hands Bob a note from Patrolman Wells.  In
O. Henry's typical ironic reversal, this note informs Bob that he was there at the
appointed time, but when he struck the match, Wells recognized the man wanted in
Chicago.  But, because he did not have the heart to arrest his old friend, he went to
the police department and asked a plain clothesman to do the
job.