If you take someone else's argument or even just their thesis statement?
Unless they have a really unique idea, there's not much that distinguishes a thesis statement or outline/argument of a paper.
Nothing word for word, just the idea, the concept?
Or if you take an outline of their paper and then redo the research? Is that plagiarism?
Another variation, what if you took someone's paper, reused their thesis, argument, even their sources, but checked the sources yourself, added a few more source or even a little extra argument? Everything still actually done by you, but pretty closely following someone else's paper? All your own writing, all your own source checking, but mainly a paint-by-numbers project. Is that plagiarism?