The theme that most people refer to when discussing Saki's "The Open Window" is deception. The goal of young Vera is to deceive Mr. Nuttel into believing that the men returning from the hunt that day are ghosts. She does this fabulously because Mr. Nuttel is a weak-minded bore who has no imagination. She is also good at what she does, which is the following:

"Romance at short notice was her specialty."

Vera deceives her aunt and uncle at the end of the story as well by telling them a believable lie about Mr. Nuttel who runs out of the house when he believes her supernatural tale. Apparently, deceiving any and all adults can be a theme as well. 

The characters, setting, and plot all relate to the theme of deception as Vera manipulates her knowledge of these three elements to create a believable practical joke. First, Vera uses what she knows about the adults to create her story (characters). For example, she finds out that Mr. Nuttel doesn't know anything about the Sappletons, which gives her an upper hand and puts him at a disadvantage. Then, she manipulates what she knows about the way things are done at the house on hunting days to support her tale, such as leaving the French windows open for the three men. Next, Vera uses the open window as a symbol of grief and suffering to manipulate Mr. Nuttel's feelings (setting). Finally, she combines the adults' typical behavior on hunting days with her storytelling skills and quick wit to create a suspenseful and believable tale (plot). By aligning the timing of her story with the men's return from hunting, along with her shocked and credible response, Vera creates enough suspense and support to solidify her deceptive ends.