icarusancalion has done a very good write-up on the difference between plagiarism and fanfic.

Fanfic can sometimes walk a very narrow tightrope, that question 'can fanfic be considered as original works or is it just copyright rip-offs?' becomes even more obscured when people plagerize. Whether it's a well-known published work or someone else's fanfic.

Go read Icarus's little ok; not so little essay.