What is the theme of "The Open Window"?
Out of a variety of themes in the story, the one that dominates is manipulation versus frailty. The story is possible thanks to the manipulative nature of Vera, a girl who is fifteen years of age, but still quite "self-possessed" and in control of herself and her circumstances.
Her curious and yet strong personality is what makes her decide to employ her storytelling skills to mentally disturb house guest Framton Nuttel, a man who is obviously awkward and suffers from a nervous condition. The fact that she does this on purpose shows that she feels quite in control of the man, and also that his condition is noticeable enough that she feels piqued to challenge it.
Vera's manipulation is strong, regardless of the fact that she is younger and expected to have less experience in life than Framton. Yet, his frailty of character and his lack of mental stability puts him in a susceptible position, where he is at the mercy of what the young girl says to him. Therefore, his gullibility and feeble nature act against him.
Sadly, his lack of self control will always make Framton prey for anyone whose personality traits are contrary to his own. Anyone who is both "self-possessed" and mischievous enough to want to cause harm would immediately become an adversary to Framton. Since he goes from bad to worse in the story, one may suppose that things will only go worse for him in life from there on.