Can the Following be Considered Plagiarism?

Let's say I'm writing a short story for an English class and I come across the ending words from Isaac Asimov's "Foundation":

"Let my successors solve those new problems, as I have solved the one of today."

I decide they perfectly express the ending to my short story and I change it to the following and submit the paper for a grade:

"I leave it to those that follow after me to deal with those new issues, because I have dealt with the one presently at hand."

Is that single alteration plagiarism? What if I applied the same technique to an entire short story written by someone else and completely changed the character of their story sentence by sentence. Would that be plagiarism?