The narrator claims that he has undertaken a mission to un-sully the reputation of once-acclaimed author F. Payne Winchester (quite obviously F. Scott Fitzgerald), now deceased. He is contacted by an agent who wants some help with a screenplay by Winchester's former lover, Cecile. He reads the manuscript but says it is all wrong, and that Cecile's obviously been too influenced by all the Hollywood lies about Winchester without even questioning them herself. He decides to write the story himself, and in the process seduces Cecile (now married and a mother) and manages to obtain Winchester's papers, which he chooses to destroy. Now that he has put Cecile in a compromising position and has destroyed all other evidence, he is safe to publish his version of the story, which he says ends with a great message of hope for the people.
Here's a quote that I particularly liked:
We drove all over the country to studios and night clubs and to see some of her former friends (who mostly tried to cooperate but did not have the feel of what I was after)....
They didn't have the feel of what he was after because he wasn't after the truth. He was after a story and success, and was willing to sacrifice the truth to get it.
What's interesting is that the narrator is consistent in presenting himself to the reader as doing a good deed for both the heroic Winchester and the the public, while we can easily see through to his real motives. The whole story is a lie about a lie.
What's also interesting is that this lie-about-a-lie is partially true. F. Scott Fitzgerald had ruined his reputation by the end of his life. His contract with MGM was not renewed, presumably due to his alcoholism, and he was having an affair (although his wife, Zelda was in an asylum on the other side of the country). When he died, "The obituaries were condescending, and he seemed destined for literary obscurity" (from "A Brief Life of Fitzgerald").
Although the Fitzgerald "revival" was probably sparked by Arthur Mizener's biography, The Far Side of Paradise (1951), by all accounts it did not whitewash Fitzgerald's life, however much later critics revived his literary reputation. However, the film Beloved Infidel ("a touching look at the relationship of Hollywood gossip columnist Sheila Graham & F. Scott Fitzgerald")—I should note that I haven't actually seen the movie—seems to be exactly what Cassill's story describes.