Copyright is not about protecting ideas, that's for intellectual property.

If you're not trying to profit from it, you can pretty much write what you want.  
But the minute you try to make cash from base don someone else's work, watch out.

My script stab at a feature was a "training wheels" exercise.
It was a "re-imagining" of an old screenplay.
The process helped me hone my skills while giving me a road map.

For something like that, using copyrighted material for private use is acceptable.
Some folks call it "fan fiction" and there's lots of it out there.

But for something you want to sell, make it original or get the rights.

Hope this helps.

Regards,
E.D.