You'll want to choose a short story where the theme and plot are closely related.  For example, "The Most Dangerous Game" is a good choice.  You could discuss how the plot of a man landing on an island and becoming prey relates to themes about the importance of human life.

Interestingly enough, there's a work by Ernest Hemingway titled "A Very Short Story" that may be helpful to you in completing this assignment.  It is indeed very short, but it also involves a very major change in the relationship between two characters. The plot of this story would be easy to summarize, and the whole "point" of the story seems to be to trace the evolution of a couple's relationship. You should be able to find the story easily by searching for it on Google.

When a student composes a thematic analysis of a story's plot, she explains how the theme is developed through the use of the elements of the story such as characterization, setting, point of view, and, of course, the sequence of events.  For instance, in Kate Chopin's "Story of an Hour," the theme of feminine repression is developed through the characterization of Mrs. Mallard, the reportorial passive voice of the narration, and the diction of Chopin.