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 two events represent the rising action and climax of the narrative, as they involve key developments in the story leading to the resolution.