Why does Framton Nuttel visit Mrs Sappleton?

Framton Nuttel is actually calling on both Mr. and Mrs. Sappleton, but Mr. Sappleton happens to be out shooting with two of his wife's brothers, so Framton will have to wait to meet the men. He only meets Vera at first and then her aunt. He is a shy man and doesn't enjoy making these visits to complete strangers. He is only doing it because his sister insisted on his making an effort to have a little social activity while he was undergoing his rest cure in the country.

"I know how it will be," his sister had said when he was preparing to migrate to this rural retreat; "you will bury yourself down there and not speak to a living soul, and your nerves will be worse than ever from moping. I shall just give you letters of introduction to all the people I know there. Some of them, as far as I can remember, were quite nice."

In those Victorian times there were no telephones. Presumably Framton would just have to appear on the doorstep and hand his letter of introduction to a maidservant. Mrs. Sappleton was not quite prepared to receive a visitor, so she sent her fifteen-year-old niece to substitute for her while she fixed her hair and perhaps changed into a different dress. This gives young Vera an opportunity to practice being a hostess, but she takes a mischievous delight in practicing in an entirely unorthodox fashion. 

Vera is bored to death in this household. She sets Framton up for the scare of his life because she knows exactly what her aunt is going to talk about when she appears, and she even knows that one of the returning hunters is going to sing, "I said, Bertie, why do you bound?" These three men, according to the girl, were supposed to have died three years ago when they were sucked into a bog on the moor. Vera introduces a note of spookiness by telling the nervous visitor:

"Do you know, sometimes on still, quiet evenings like this, I almost get a creepy feeling that they will all walk in through that window."

Mrs. Sappleton is such a rattlebrained woman that it is easy for Framton to believe that her expectation of the three hunters at tea time is a sign of the insanity purportedly brought about by their deaths. Framton never does get to meet the men of the family because he is sure they must be walking dead. The fact that they are all carrying guns makes them even more frightening. What convinces Framton that these men returning towards the open window must be ghosts is the faked look of horror on the face of the girl who had been depicted as "very self-possessed" up to now.

Mrs. Sappleton is the first to see the hunters approaching.

"Here they are at last!" she cried. "Just in time for tea, and don't they look as if they were muddy up to the eyes!"

Framton shivered slightly and turned towards the niece with a look intended to convey sympathetic comprehension. The child was staring out through the open window with a dazed horror in her eyes.

Framton's sister's letter of introduction has just the opposite of the intended effect. Instead of meeting a nice country family, Framton believes he has entered a house of horrors and flees for his life. No doubt he will not be presenting any more letters of introduction to strangers.