Finally they were ready shows that the setting affects the plot of the story.
Step-by-step explanation:
Plots in a story are the literary devices that show the events that make up a story. It describes the events of the story or a major part of the story. They occur in a particular sequence or a pattern which shows the pattern of the story.
In this passage, the part that shows that the setting is affecting the plot of the story was finally, they were ready. This means that the characters of the story were ready and they were about to leave for the picnic. This shows that a new sequence of the story is about to start.