Inspired:
"Wow. The idea of being trapped in a virtual MMORPG is very interesting! Sadly, this anime is very horribly-written and turned out very badly. Oh! I know what interesting conflicts that might happen if some people were trapped in a virtual MMORPG like the ones in that anime."
I'm talking about an anime called Log Horizon. It's set in a theme similar to Sword Art Online (trapped in an MMORPG), and seems to be inspired by it and my favourite MMORPG, Ragnarok Online, at some points. What doesn't make it a plagiarism is that it has original conflicts, story line, etc. Watch the two and you will get it.
Plagiarism:
"Geez. This series is cool, but I wouldn't have set it that way. I will make my own series with similar character, setting, and stuff, making it run exactly on how I want them to."
Example (an extreme one):
Already see the differences? In the first example, a new and fresh idea is triggered by another story. In plagiarism, you're merely rewriting an already existing story with your own twists, usually to get readers or just to fulfill your own fantasy. You rewrite the story of Romeo and Juliet (a couple with different social statuses and both of them end up *spoiler* dead, bla bla bla) but with a modern high school as its setting (because I think it's so original and fresh), or something along that line. That's plagiarism, as well as those ponified story. None of the essential elements changed. Only this time it's in Equestria, with some extra hoof-and-hay minor conflicts.
Well, duh. Show your readers, obviously.
You can't expect to introduce them by, "Hi, I'm Voldemort. I'm an awesome wizard who can do some tricks and kill you with a swing of my wand. I'm very dangerous, I'm telling you, so you must fear me. Thanks, guys." We got the idea that this Voldy is the villain as soon as one of the characters told another character about whom he killed, or showed their fear to the Dark Lord by not daring to say his name, etc. At this moment, we already have the idea that Voldy is the villain. I mean, killing is clearly an evil thing, but we haven't got the idea about how dangerous, cruel, and scary he is, or why, so J.K. Rowling, as a kind author that she is, showed us that by making Harry to have that horrifying duel where Cedric dropped dead.
You don't need to think about the way your villain shows up first, like, "Should he show up with an evil black coat or a skull walking-stick?" Show us why one is evil, or why should we believe that one is evil. Audiences tend to believe what's shown on the stage (ex: a man brutally chewing a baby), so that's where you play the tricks and illusion.