Are you really here to watch my soccer game, Mrs. Weigand?” I asked shyly.
How does the sentence help to develop the story’s plot?

The given sentence is a first person point of view statement where the protagonist is kind of embarrassed about the fact that Mrs. Weignad saw him in the soccer game.

Explanation:

This is a key sentence here, because the plot can take any turn from this point. Mrs. Weigand, may be a sweet lady or an inquisitively notorious lady which may cause trouble to the protagonist as the plot goes. Anyway the given sentence has a potential to change the course of the plot.