> only the plot is copyrightable

But the plot can't be copyrightable, as the copyright applies only to a tangible representation of an idea (e.g. written text), and not to an idea itself.

Code is one thing, but what about writing? There is no 100% foolproof way to identify content written by LLMs, and human writing routinely gets incorrectly flagged as such. If I write a book, and a checker says that it's written by LLM, is it automatically in the public domain?