Out of curiosity, I've clicked on some fan stories related to books and movies I've never read nor seen.
So many of those stories assume that the reader is already very familiar with the book or tv show in question, that unless the person has already seen them, they don't "get" the story.
For stories related to books or movies I have seen, I must admit I've seen very little relationship to the original book or movie except the title and a couple of characters.
I haven't use the calibre plugins for fan fiction yet. But I do notice that there is no "apprentice" needed for using them, so that is good for me if I am going to start reading more fan fiction. It tells me that it is at least legal to read it