Gortsby has been having a brief conversation with a young man on a park bench. The young man has taken the place of an elderly gent, whom Gortsby imagines has gone home to what he presumes to be bleak lodgings or a home where he's ignored and insignificant.
The young man spins Gortsby a tale about being stranded in London without any place to stay. As part of his elaborate story, he tells Gortsby that he has no money left on account of spending his last shilling on buying a drink and some soap. But as the young man appears not to have the soap on him, Gortsby thinks he's being taken for a ride by a con-artist who just wants to get some money out of him. But after the young man gets up and walks away, Gortsby discovers a bar of soap on the ground next to the park bench. It seems that the young man was telling the truth, after all.
So Gortsby anxiously sets off to find the young man. After finally catching up with him, he hands over the bar of soap and gives the young man some money. As one can imagine, the young man is highly delighted with his sudden unexpected windfall. When Gortsby returns to the park bench, he sees the elderly gent he'd sat next to earlier. He appears to be looking for something. When Gortsby asks him if he's lost anything, the old man replies
Yes, sir, a cake of soap.