The themes in The Open Window revolve around appearances and reality. Consider that the story that Mrs. Sappleton's niece shares with Mr. Nuttel and how she appears genteel and cultured, but in reality she is vicious. The open window becomes the vehicle for which she offers an alternate view of reality. In the end, what is real?
Another theme is that of deception and lies. All of the action is spurred on by the deceit revolving around the window and the fact that it is open. The underlying theme in this action is that the author, himself, is asking whether the teller of the tale's motives were that of a cruel and bored woman, or merely done as a harmelss prank. Deeper still is Saki's question of the motives behind all storytellers.