There are some stories that are so classic and well loved that they've been used as the basis for other stories. How many times have you encountered "Cinderella" or "Beauty and the Beast" in some form or another? The elements of these legends and fairy tales are timeless and we can't help but want to incorporate those elements in our own stories.
But there is a line between blatant copying, retelling, and taking inspiration from other sources. What is this line, and how do we not cross the ones we don't want to? It's difficult to see the lines, especially when you are aware that what you're doing is a retelling.
The first step to understanding this line is to know the definitions of copy and retelling. These definitions are by no means "academically correct", but are here to give a general sense of them
A retelling is a story in which the main ideas, characters, setting, etc. have been reworked in order to establish new meaning, theme, and/or arguments, or to complicate or simplify the original story. For example, there are many traditional versions of 'Little Red Riding Hood'; some are meant to warn against strangers or discuss puberty, and some have Little Red saving herself as opposed to a woodsman coming to save her.
A copy of a story may have different names for characters, races, lands, creatures, etc. but themes/meaning, complexity, arguments, general setting, sentences/paragraphs, and/or plot points are exactly the same as another work. Often a copy has multiple or all of the above grievances working against it, and often the creator will try to pass the copy off as original. If it fits the definition of "plagiarism", it's a copy.
Between these two is the elusive "inspired by" category, which is much like using quotes in your school paper and having to put the credits in the footnotes. This is when there are obvious allusions to other works, even going insofar as to copy, but ultimately it is considered its own work. For example, the 1987 movie "Willow" is inspired by the works of J.R.R. Tolkien almost verbatim: there are little people who live peacefully by themselves and live hidden from the rest of the world, there is a war between the big human creatures, we have a supportive best friend, a group of fellow little people trying to get something out of their home, a golden-haired elf/fairy/goddess character who gives them something special/magical, a vagabond character who ultimately saves and helps them (also the love interest)... we even have two characters who know nothing, do nothing significant, and are there for comedy relief (I'm looking at you, Brownies). However, this is not a retelling, as there is nothing original about its theme, meaning, or complexity, and yet it is not considered a copy either. This is because the movie gives enough nods to the original work and admits it.
Of course, admitting that your work took inspiration from another isn't enough to get away with a copy. There must be enough original content where it isn't considered a copy. This is where things get tricky, complicated, and subjective. How much does the audience see as original/inspired by, and how much do they see as a copy? How much originality does an audience want from your work? For those more well versed in any particular genre/medium, they may be unwilling to see something as an inspiration and may see it more as a copy. I, for example, didn't like "Willow" because I saw it as a blatant copy, and I really hated that fact. Why make "Willow" when you could just make "The Hobbit" or LOTR? (PS. the answer is "budget"). However, there are always those who LIKE the same thing. That's the reason why many Rom-Coms and Action movies are the same: people LIKE the familiarity.
This, of course, brings about questions of "originality". To be clear, there is nothing new under the sun. Everything we know has ultimately taken some inspiration from something else. Humans are pattern-seeking creatures, and we imitate those patterns whether we meant to or not. A great example is this video:
It's why people who have never written a book or directed tv/movies or acted in their entire lives can make judgments about other people's work: we've seen and remembered enough to get a sense of what is good and what is bad. We feel the patterns and we make judgments based on our experiences with those patters. If we feel, based on our individual experiences, that a work is too much like something we've already seen, we're more likely to see the work as a copy rather than an inspiration.
The things that we find to be "original" typically have elements that come from a deeply personal place as a creator. To take another Tolkien example: the creation of Hobbits was based on the English people, and how Tolkien saw and experienced "Englishness" in his opinion. As someone who lived nearly 3 years in London, I can see how the smoking, drinking, and love of gardens really shines through. The society of Hobbits is what I imagine a country village might have looked like in the 17th or 18th centuries (possibly earlier). The world of Middle Earth doesn't try to be anything other than English in its origins, which is all because Tolkien was English, studied history and languages, and lived in one country or most (if not all) of his life. Much of the "original" lore we find in this universe are reworkings of older myths: faeries and elves being able to cross worlds (water being chief among their means of travel), little people living in hills or mounds, wizards in general, powerful magic artifacts, etc. If Tolkien hadn't written it, the LOTR universe would have looked and felt VERY different.
That said, some people like the idea of taking other people's work and trying to make it their own. There's nothing wrong with this, and in fact it can be very powerful. In my opinion, it is always better to do a retelling than take inspiration. Not only is it easier to "get away with" as far as originality goes, it allows for your own voice to shine through all the dressing of a story. When you're not concerned about creating a world from scratch, you can focus on your message and meaning; think deeply about what the original story implies, what you agree and disagree about, and how it can be altered or improved.
The bottom line is that the CREATOR is the difference between a copy and a retelling. When a creator is trying to tell someone else's message through the same means, it is a copy. It is truly "uninspired" because there's not much differentiating between the original work and the new one. When a creator has something else to say, that is when an "inspired by" or a "retelling" story can shine and become its own work.