The details that would be included in the middle of the plot diagram are:
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After listening to his story, Gortsby accuses the young man of lying because he cannot produce the cake of soap that forms part of his story.
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After the young man leaves, Gortsby finds a cake of soap lying on the bench and believes that the young man left it there.
These details represent the rising action and climax of the story, as they build tension and lead to the significant turning point of the plot.