The short story "The Possibility of Evil" by Shirley Jackson begins with Miss Adela Strangeworth taking a walk on a clear fresh summer day. She is proud of her house, her roses, her family history, and the fact that she knows everyone in town. She even delights in telling passing tourists of her family lineage and the town's history. She is a model citizen, admired and respected by all, at least in her own opinion.
However, Miss Strangeworth has a sinister side to her character. Although she sees herself as faultless, she imagines potential evil in everyone around her. As a way of correcting them and pointing out their faults, she sends cruel anonymous letters to the townspeople. These letters upset them and subject them to fear and worry.
On the bright summer day with which the story opens, Miss Strangeworth writes three more of these cruel letters when she returns from her walk. Later she goes to mail the letters, but she waits until dusk:
"so she could reach the post office just as darkness was starting to dim the outlines of the trees and the shape of people's faces."
She wants her disturbing notes to remain secret.
Children are playing near the post office. When she pushes the letters into the slot on the post office door, two fall inside and one falls outside. Miss Strangeworth does not notice, but a boy named Dave Harris sees the letter fall outside. At first he calls after Miss Strangeworth to let her know that she has dropped the letter. Linda, a girl he is with, tells him to throw it in the post office.
However, the Harris boy notices that it is addressed to Don Crane. He decides to take it over personally as he and Linda will be passing Crane's house anyway. He thinks that he is doing Miss Strangeworth a favor because it might be a check, good news, or something else that Crane will be happy to get. He never suspects that the letter might be nasty. In this way, Miss Strangeworth is exposed as the person who has been writing the letters that have troubled the town, and in anger, Don Crane destroys the roses outside her house that she values so highly.