While we editors at enotes do not write essays for students, we can certainly offer suggestions. With the theme of the deceptiveness of eternal circumstances, you will want to demonstrate in your essay how this theme is developed. So, if you are writing a five-paragraph essay which is usually standard for high school, your thesis should contain three opinions which will be developed into topic sentences for the body of the three body paragraphs.
Of course, the external circumstance involves the bar of soap which Norman Gortsby ironically believes is the proof of the young man's tale of woe. This external circumstance is also the tool for Saki's satire on the character Grotsby, who prides himself on being a good judge of people, especially those who come out in "the hour of the defeated." As the story turns out, it is Grotsby who is defeated, not the young man, through the ironic twist of the elderly gentleman's having lost a bar of soap--the external circumstance. Grotsby is guilty of his criticism of the young man:
In his particular line genius certainly consists of an infinite capacity for taking precautions.
For your introduction, you may wish to refer to the site below which gives you a short biography of Saki, describing how he liked to satirize the audience to whom he wrote. In "Dusk," he sets up his satire based upon the external circumstance involving the soap. This circumstance becomes deceiving to Grotsby because in the dusk, the shadowed gloom that disguises things, he does not notice the soap. The three points of the thesis can be an explanation of why he does not notice the soap. In so doing, you may wish to consider the fact that he appears rather smug about his judgments of people, he perceives himself outside the group of the "defeated" [his first name Norman suggests conquerors] who sit in the dusk, and unwittingly, he fulfills his "mood to be counted among the defeated" by regaining faith in the young man.