What effect does the dramatic twist have on Framton and the reader in "The Open Window''?

With her
self-confidence and poise and the use of the open window as the framing ( intended!) device for
her tall tale, both Framton and the reader are taken in; there is no question. Thus, believing
that Vera's narrative is a tragically veritable history of Mrs. Stappleton's family, Framton,
along with the reader, feels pity for the deluded Mrs. Stappleton when she exclaims, "Here
they are at last!" 

But, of course, it is not Mrs. Stappleton, but
Framton, along with the reader, who are deluded. Poor Framton, is terrorized by the look of
horror on the girl's face, for he does, indeed, believe...