Is it plagiarism to reference a fictitious source?
For example, Merriam–Webster defines plagiarism via to plagiarize, which it defines as:


  : to steal and pass off (the ideas or words of another) as one's own
  : use (another's production) without crediting the source
  intransitive verb
  
  : to commit literary theft : present as new and original an idea or product derived from an existing source


This does not include inventing a quotation, which is in fact sort-of the opposite of plagiarism: passing off one’s own idea as somebody else’s. Other dictionaries agree on this and so does my understanding of the word plagiarism.

Moreover, defining plagiarism so broad makes the term rather useless and almost equivalent to the umbrella term academic misconduct. The reason why we have a word for plagiarism is to differentiate a specific kind of misconduct, not a specific severity.


  The conflict is this: the student isn't citing these texts directly, but they are referencing finer plot points and characters. To me that's dishonest and shows a lack of integrity. I'm grading students on a rubric that awards points for organization, analytical treatment, and language use. If I treat this as academic dishonesty maybe I knock the person's grade down in the rubric criteria related to analysis, but if I treat it as plagiarism I'd give the student a zero.


I fail to see why you would be more lenient about academic dishonesty than about plagiarism. I don’t fully understand what you mean by “referencing finer plot points and characters”, but I would classify what you are describing as fabricating evidence, which is roughly as grave as plagiarism. I say roughly, because I see no point in ranking the severity of those misconducts in general and the severity distributions of individual instances of those misconducts strongly overlap.

What is important at the end of the day is whether you are reasonably convinced that the student in question did not just work sloppily, but intentionally deceived the reader (i.e., you). The aspect of intention alone suffices for awarding them zero points, in my opinion.