At what point is an idea theft? At what point is it borrowing? At what point is it influence?
Lets take a popular show/movie whatever. Or better yet, lets take something that hasn't been around in awhile. I mean, everything old becomes new again, and if I'm the one who is making it new, then that means I'll be on the ... top of the wave thingie.
How about, Japanese animation? They have tons of stories that used to be popular. So lets see.... Sailor Moon!
If I were to write a story about an ancient moon princess, who is reincarnated on Earth, and gains super powers by transforming into a skimpy outfit, along with her 4 other friends, all with different personalities, to fight the same evil that destroyed her kingdom eons ago, I'd be stealing the story of Sailor Moon.
What could I remove from that equation, that would still be Sailor Moon (teenage girls with powers fighting evil), and not be Sailor Moon?
We could change the basis of the powers, the number of friends, and get rid of the reincarnation stuff. In this case we would lose the planet motif, and make 3 girls (the main character with her two best friends), we'd pick our favorite 3 personalities, and completely redo who the main villain is. Also, set it in the present, or even the slight future.
Now a great deal of what made the Sailor Moon setting is gone, and what we have left is the personalities of the main cast, and how they influenced each other.
If the setting was not modern, but lets say fantasy, we might be in Magic Knight RayEarth. If there were only the two girls it might be Pretty Cure. As is, it might be Powerpuff Girls Z.
Are we now so far removed from its origins that we are now writing under the influence of having seen Sailor Moon, or are we still stealing?
I leave you with this interesting link.