In "The Open Window" how do you know Frampton Nuttel has never met the Sapplestons before his visit?"The Open Window" by Saki
In Saki's craftily executed frame story, not only is the main character, Framton Nuttel, taken in, but so is the reader. Perhaps Saki's greatest story, "The Open Window" begins with the frazzled Framton Nuttel arriving at the Sappletons, whom his sister knows, with a letter of introduction which she has provided him because she worries that he will "bury" himself in his rural retreat meant to be a nerve cure and not speak to anyone.
As it turns out, Framton is held captive by the niece of Mrs. Sappleton, and she is a young lady whose talent lies in blurring the line between imagination and reality so well that poor Framton is terrified after gullibly believing the niece's tall tale about her uncle and cousin. So, rather than working a "nerve cure" for Framton Nuttel, she has unnerved the man so much that he jumps up and flees without even saying goodbye.