The topic sentence will depend on the angle or theme you are working with. For example, consider the notion of sanity. Framton Nuttel ("nuts" - crazy) was trying to cure his nerves with a trip to the country. Is he really crazy or is he simply a hypochondriac? And if his nervous condition is of his own doing, is Vera's behavior more "insane" because she purposely manipulates other people with her lies? This type of thematic analysis might use a topic sentence as such: "In this story, Saki shows that there is a fine line between sanity and lying." 

Or, you could focus on the "fine line" between lying and storytelling. Is Vera's lying really evil or is it a harmless prank? Note that this is a short story of fiction. Vera is doing what Saki is doing: making things up. Vera's story doesn't truly threaten anyone. Framton's nervousness puts him in a position to be scared by the story. If what Vera does is so bad, then can or should we hold all writers accountable for their made up stories? This seems to be an untenable argument, but it is interesting to consider the differences and similarities between fiction and lies. This would suggest a topic sentence(s) such as: "Saki makes things up. Vera makes things up. Is the reader correct in denouncing Vera and celebrating Saki's imagination?"