The plot centers on the peculiar fact that Button is born as an old man and ages backwards. There is never any explanation for why this happens, which to me makes this story an out-and-out fantasy. Fitzgerald merely asks, "What if someone was born old and got younger the longer he was alive?" In other words, he speculates on the effects of something that could never happen in real life. Unlike his novels, Fitzgerald sometimes turned to spec fic and even mysteries in his short stories. What's interesting is this quote about his "magazine fiction:"
"I have asked a lot of my emotions, one hundred and twenty stories. The price was high, right up with Kipling, because there was one little drop of something not blood, not a tear, not my seed, but me more intimately than these, in every story, it was the extra I had. Now it has gone and I am just like you now."
— from "Our April Letter," The Notebooks of F. Scott Fitzgerald.
According to the University of South Carolina website where I first found the quote, for a lot of his career, Fitzgerald supported himself on his short fiction sales, but "viewed his commercial stories as a drain on the creative energy he required for his novels."
I find it interesting that he saw the stories as commercial only, but even more interesting to me is that today, he would probably not bother with them because it is much harder to make money from short stories than from books. Most of the magazines he published in are gone. I wonder what his work would have been like in today's market?
Now there is some speculation!