their history. Therefore, he has no reason to question when Vera tells him that Mrs. Sappleton's husband and younger brothers were lost in a bog and never found—or that Mrs. Sappleton expects to see them walk through the open French window (we would call it a French door).

Vera is a good storyteller. She offers such a detailed description of the husband and brothers that Nuttel believes he is seeing ghosts when they come through the window.

To me, there are a couple reasons why Framton Nuttel falls for Vera's story.

The first has to do with Vera.  She is clearly a very good actress and that is not something you would expect from some 15 year old girl -- you wouldn't expect her to come up with a wild story and then tell it so seriously.

But the second has to do with Nuttel.  We can see from what we are told of him that Nuttel is not the most sophisticated or the most social person around.  His sister predicted that

you will bury yourself down there and not speak to a living soul, and your nerves will be worse than ever from moping.

If he is that sort of a person, he may not really be perceptive enough to realize when he is being tricked.

So I think it's a combination of Nuttel's gullibility and Vera's believability.

Another thing that makes it easy for Vera to fool Framton Nuttel with her ghost story is the fact that the whole family leads such a monotonous, routine existence. Vera knows in advance exactly what her aunt is going to say about the open window and exactly when the three hunters will return for tea. She even knows that Bertie will be singing the same song, with the refrain:

"I said, Bertie, why do you bound?"

It is because of the monotony of her own existence that this bright, imaginative fifteen-year-old girl decides to inject a little diversion and humor into her dull family.

Vera also finds it easy to fool Framton Nuttel because she has such a vivid imagination. She can invent the wildest stories spontaneously and make them entirely convincing. She may have been toying with the plot for her ghost story on many dreary evenings while waiting for the three hunters to return through the open window; so she would have the details worked out before the right victim ever appeared. There are several clues to suggest that she had recognized Framton Nuttel as the ideal listener to her gruesome story.

"Do you know many of the people round here?" asked the niece, when she judged that they had had sufficient silent communion.
"Then you know practically nothing about my aunt?" pursued the self-possessed young lady.

She has to be sure that the visitor is totally ignorant about her aunt and the other members of her family. Saki has created the ideal victim for this mischievous girl. Nuttel is a complete stranger to the area. He is just now beginning to try to make a few acquaintances with his letters of introduction from his sister. 

Vera very cautiously and cunningly makes sure that Framton Nuttel is a stranger in this part of the English countryside and would not know anything that would contradict the totally fictitious story she plans to tell him. First she asks, "Do you know many of the people around here?" This is just the sort of question a hostess might ask a visitor like Framton, who is presenting a letter of introduction. 

"Hardly a soul," said Framton. "My sister was staying here, at the rectory, you know, some four years ago, and she gave me letters of introduction to some of the people here."

It should be noted that the author Saki has invented exactly the kind of character who will be a perfect victim for Vera's practical joke. Framton Nuttel is a stranger to the area, he is suffering from a nervous disorder, and his doctors have advised him to rest in the country. As he will tell Mrs. Sappleton shortly later:

"The doctors agree in ordering me complete rest, an absence of mental excitement, and avoidance of anything in the nature of violent physical exercise." 

Ironically, Vera joke will produce just the opposite results, including violent physical exercise when he flees the house and is last seen running for his life down a country road.

Vera continues fishing for information. 

"Then you know practically nothing about my aunt?" pursued the self-possessed young lady.

"Only her name and address," admitted the caller. 

This is enough to satisfy Vera. She doesn't have much time before her aunt will appear and take over as hostess. The girl quickly launches into her story about the three men who went bird-hunting three years ago and were sucked into a bog. If such a tragedy had really happened it would be known all over the region and Framton would have heard about it. Vera has to establish that he is clueless--and Saki has to establish the same thing for the benefit of his readers. Only a complete stranger would do for the role that Framton will play, and only a man who is suffering from what we nowadays would call a severe neurosis would react as spectacularly as Framton does when he sees three armed men approaching the open window in the deepening twilight.

One of the approaching men establishes that they are indeed the three hunters who were supposed to have died when they were sucked into the bog. He breaks into the song which Vera had told Framton he always sang when he returned home at teatime.

Noiselessly they neared the house, and then a hoarse young voice chanted out of the dusk: "I said, Bertie, why do you bound?"

And when Framton turns to look at Vera, he sees that: 

The child was staring out through the open window with a dazed horror in her eyes. 

That does it!

Vera knows all about Framton's nervous condition and that he's spending time in the country as a rest cure. She knows this because Framton's sister provided him with introductions to a number of families in the area, one of which is the Sappletons. Now, most people would go out of their way to make sure not to do anything that would excite Framton's frazzled nervous system. But not Vera. As she's such a mischievous young lady with a vivid imagination and a gift for making up stories, she can't resist this golden opportunity to have a bit of fun at poor old Framton's expense. Framton inadvertently makes things easier for Vera by telling her that he doesn't know anyone in the local area. This gives Vera the freedom to make up a shaggy dog tale about Mrs. Sappleton's husband and her two younger brothers.

Frampton Nuttel is in the country visiting people that he never met as a remedy for his nervous condition.  When he arrives at the Sappleton home, he is greeted by Vera, Mrs. Sappleton's niece. 

The only thing that Vera finds out about Mr. Nuttel is how many people he knows in the area.  She asks him if he is familiar with the area, because she would not be able to tell her tall tale if Mr. Nuttel was familiar with the area in the country and its inhabitants.

Once Vera is told that Mr. Nuttel does not know anyone in the country, but has come by way of his sister who is an acquaintance of her aunt's, Vera then goes into the story about her Uncles being lost in the bog.

The beauty of Vera's explanation of Framton Nuttel's unusual behavior is that it is so strange and exotic that it seems impossible that a girl her age confined to a country home in England could possibly have made it up. And yet it seems plausible. In a country like India where humans die of starvation on the streets every night and are carted away in the morning, there must be dogs who have an even harder time surviving. Dogs have a natural instinct to roam together, so homeless dogs would travel in packs and could be dangerous, especially if a person happened to be alone in an isolated place. Indians put up with a lot from animals because they do not believe in killing living creatures. There are some religious people who wear masks over their mouths to keep from accidentally swallowing an insect. And the toleration of monkeys is well known.

It seems almost necessary for Vera to tell her story about the pariah dogs at the end, since someone would naturally want to know why the visitor suddenly jumped up and went running out of the house without a word of thanks or goodbye. One of the three returning hunters has to say something in order to establish that they are living men and not ghosts. Vera's Uncle Sappleton asks:

"Who was that who bolted out as we came up?"

And Vera casually offers her explanation:

"I expect it was the spaniel," said the niece calmly; "he told me he had a horror of dogs. He was once hunted into a cemetery somewhere on the banks of the Ganges by a pack of pariah dogs, and had to spend the night in a newly dug grave with the creatures snarling and grinning and foaming just above him. Enough to make anyone lose their nerve."

The girl is bored. That is why she decides to stir up some excitement. "The devil finds work for idle hands." She must spend much of her time reading books. And since she is bored with her life, she probably favors escapist literature. She must have picked up the anecdote about the pariah dogs from a book about India. It really is a vivid picture, and it probably would explain how a man might develop "a horror of dogs."

Vera is able to lay out a trap for Mr. Nuttel because when she first meets him she ascertains that Nuttel doesn't know anyone in the neighborhood. Nuttel explains that his only connection is "my sister was staying here, at the rectory, you know, some four years ago, and she gave me letters of introduction to some of the people here" (Saki). After confirming that Nuttel knows almost nothing about Vera's aunt outside of her name and address, Vera is able to lay a trap for Nuttel.

Vera notes that Nuttel says his sister was in the neighborhood four years ago. Therefore, she makes up a tragedy that happened three years ago, after Nuttel or his sister could have had any knowledge of the neighborhood.

Vera indicates the open window to Nuttel and says that he must think it strange to have an open window in October. She then tells him that three years ago, her uncles went out shooting and were swallowed up by a bog, but the "bodies were never recovered" (Saki). She tells him, "Poor aunt always thinks that they will come back some day, they and the little brown spaniel that was lost with them, and walk in at that window just as they used to do. That is why the window is kept open every evening till it is quite dusk" (Saki). Vera plays the part and acts as though this story that she is telling saddens and disturbs her.

Nuttel believes Vera, and when the aunt appears and says that her husband and brothers have gone out shooting but should return for tea, Nuttel is increasingly alarmed that the aunt is not in her right mind. He believes Vera's story that the men died, and so he thinks that the aunt must be deranged. When Vera's uncle and her aunt's brothers do indeed arrive home, Nuttel has become so nervous that he believes that the men are ghosts, and he runs from the house as quickly as he can.

Framton Nuttel suffers from a nervous condition and is possibly a hypochondriac. He may have a legitimate neurological condition and it may be psychosomatic. In either case, he is quite easily affected by Vera's story. Vera tells the story that her aunt's husband and brothers left through the window three years ago and never returned. They were presumed dead. Vera ends with, "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—” 

When Vera's aunt walks into the room, Mr. Nuttel is relieved. However, Ms. Sappleton then tells him she expects her husband and brothers to return soon. Mr. Nuttel is horrified by this.

To Framton, it was all purely horrible. He made a desperate but only partially successful effort to turn the talk on to a less ghastly topic; he was conscious that his hostess was giving him only a fragment of her attention, and her eyes were constantly straying past him to the open window and the lawn beyond.

He tries to change the subject and relates that he is supposed to avoid all mental and physical exertion. Then the husband and brothers return and Mr. Nuttel leaves in a hurry. But it is Vera's story that sets up his horrified need to escape. One could argue that Vera is malicious in this practical joke. On the other hand, Mr. Nuttel is just too impressionable and too gullible. What are the odds that he would arrive on the alleged tragic anniversary? He never considers that this coincidence is suspicious. When he feels uncomfortable, he starts talking about himself and his ailments. Vera's story easily plays with his vulnerable and sensitive tendencies. Vera is a gifted storyteller/liar, but Mr. Nuttel's weak mental state makes it easy for her to fool him.