plagiarism is a very tricky subject. it's not what is copied, it's how much it resembles something that has been made already. yes that's sounds like i just said two things that are exactly alike, but you need to think that MANY good ideas are already out there. hell, give me a game, book, tv show, movie, comic or anything, and i'll somehow relate it to something else. it's just HOW MUCH it resembles something else. when movie reviewers say things like "it an original blockbuster" it means that it's off the beaten path enough to make it noteworthy, but remember that no matter how far you are from that path, (an example of a horrible analogy) your still at least on the same planet that path is on.
nothing is truly original, but it can add original elements to it that make it fun and interesting.