What is the plot of "The Catbird Seat"?
There is a summary for the plot at the link below.
If you are having difficulty determining plot elements, it might help you to take notes as you read. One strategy for note-taking would be to create a time-line and fill it in as you read with events from the story. When you are done you can identify the climax, the turning point of the story, and the rising and falling action.
Here is a quick summary.
The story opens with Mr. Martin, a non-smoker, buying a pack of cigarettes. The reader then learns that this bizarre behavior is occurring because he plans to murder Mrs. Barrows, and leave a cigarette in her ashtray to fool the police.
Later, the two are talking and Marin begins smoking and drinking. Mrs. Barrows can't believe it, tells the neighbors, and when Martin denies it, she loses her faculties and is taken away.