Unless you want to use a definition such as "This story has people in it" as plagiarising the idea of "people", which strikes me as meaningless.
Which I think was the point... the concept of plagiarism can be stretched to an absolute absurd degree.
There is no copyright to a plotline, just to a specific instance of that plotline.
Perhaps I used the word 'plotline' inappropriately, I meant to refer to the actual created story, which in fact may take many different books to play out.
And again, Noey, Copyright doesn't belong to ideas at all. Only to physical results of those ideas. What is actually written down.
Almost every time a big block buster film comes out, we're likely to see a couple of mock-offs hit the theaters soon after... or in some cases, just before, depending on how long it takes the big studios to get through production. Why? Because you can't sue for copyright infringement and WIN if someone takes your ideas, changes enough elements to make it reasonably dissimilar, and gets to the finish line ahead of you. It's a screenwriters worst nightmare.