It's difficult to really get a sense for what is going on in Bob's mind as he reads the note; there are multiple angles of interpretation the text can support, with no clear answer either way. In any case, the story establishes that Bob and Jimmy had been very close friends who had arranged to meet on this particular day twenty years earlier. Furthermore, we can infer that Bob held their friendship in very high regard, given that he had kept that appointment, not knowing whether Jimmy would honor it.
At first it appears as if this reunion is carried out without any complications. However, the story's ending reveals that Bob had actually met with an undercover police officer, who had come to place Bob (a wanted criminal) under arrest. As he arrests him, he gives Bob the note, which had been written by a beat cop Bob had encountered earlier. This letter reveals that the beat cop had been his old friend, who recognized Bob as a criminal, and thus engineered the arrest.
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