According to eNotes "guide to Literary Terms," plot is "this is the arrangement of events to achieve an intended effect consisting of a series of carefully devised and interrelated actions that progresses through a struggle of opposing forces, called conflict, to a climax and a denouement."
The plot of Shirley Jackson's "The Possibility of Evil" lies in the story of Miss Adela Strangeworth. Miss Strangeworth defines herself as the ultimate authority of her (unnamed) town. Since her father built the first house in the town, on Pleasant Street, Miss Strangeworth feels a sense of entitlement and names herself (not literally) the matriarch of the town. As the matriarch of the town, Miss Strangeworth feels a sense of duty and responsibility to make known the "evil and degraded" nature of the world. Miss Strangeworth felt this sense of duty because the world was filled with people and, yet, "there was only one Strangeworth left in it."
The plot of the story follows Miss Strangeworth through a day familiar to her. It is a Tuesday, shopping day. She speaks to all who come across her path, and the narrator offers little bits of Miss Strangeworth's private thoughts on the people she comes across. Readers can see that Miss Strangeworth believes everyone to be sick or evil (based upon her remarks).
One thing which Miss Strangeworth has come to do in the past year is write anonymous letters to some of the townspeople. She cues them in to affairs, problems, issues, and secrets which she has either created or heard. She feels that it is her duty to let people in on the evils of the town. Toward the end of the story, Miss Strangeworth drops a letter and a boy (the Harris boy) picks it up and delivers it to the addressee (Don Crane). Readers can assume that the boy tells Don Crane whom the letter is from. The next morning, Miss Strangeworth receives a letter. This letter tells her of the fate of her prized roses (readers can assume that they were destroyed).
Therefore, the plot of the story lies in the tale of a woman who goes about town trying to expose the evils of the world. As a result of her exposing an evil, her most prized possession is destroyed. Ironically, it is her letters which bring about the evil she is trying to protect the town from.