I wrote a long post but the submit button ate it up. OK here we go again. This is a very relevant subject as I have both seen it happen and have it done to me. There are two kinds of plagiarisms - one where entire material is copied and the other where ideas are copied, both are wrong. To give an example of the first one; recently I read a BlogAdda WoW post that was basically a word to word copy of a short story by a well known writer (call him X). Somebody pointed it out to the author who claimed to have read novels by X but not short stories. The blogger tripped up later in a later post and claimed to have 'rebuilt' the story. I sent an email to BlogAdda containing a link to the original work. They acknowledged it and removed it from the WoW list. But the blogger has kept the story up and is still claiming credit.
I have a dog, Nawab, who has been talking to me for a while. I had him up on another website and had an audience for him before we moved to my blog. Now another blogger put an initial disclaimer on his blog 'belonged to an Ozzie friend of mine Subroto' and started chatting with Nawab. Now this is a fictional creature (clearly talking dog's don't exist, maybe they do but that's another topic) and had he asked permission to use him, I would have probably said yes. But I did find it upsetting that someone was using a character that I had created, built up an audience for and had certain emotional investment in, without asking my permission. So this kind of plagiarism is the theft of ideas, of creativity.
The first kind should never be done. The second type, well if you have the author's permission, then you can create your own stories basing them on existing fictional characters, but if you don't ask - that's not on.