I would be somewhat leery of attributing plagiarism to the duplication of plot elements. If we do that, we have to enshrine Shakespeare as the greatest plagiarist in the English language, since he only created one of the plots of his plays (The Tempest); even many of his historical plays were based on the plots of previous plays and stories about the characters. Hamlet is a prime example.

We also have to be very careful not to confuse plagiarism with copyright violation. One can easily violate copyright without committing plagiarism and vice versa.