If the idea is not your own, it is very well likely that you may end up stealing someones intellectual property (essentially, their idea).
As an example, say my idea was a movie where a computer virus that turns social media into physical manifestations of evil that are able to control technology throughout the world to end human society.
This would be my intellectual property, and because I have written it here it is technically proven to be my intellectual property unless this idea is found elsewhere with an earlier timestamp. Considering that I just made this up at the time of this post, it is still possible that another person has said it before me and therefore it is their intellectual property, though I have not stolen at, because we simply thought of the same thing.
Then there is copyright, which is a whole other story. Apple tried to copyright the word "pad", but they couldn't because it was a word. To circumvent this, they simply put the letter "i" in front of it to make iPad. This is not a real word, and therefore they were able to copyright such things. However, I believe it was a couple years ago that a company in China had copyright claims on the word "iPad" many years before Apple even made the copyright. This means that they were trying to copyright a copyrighted thing (stealing), so they were sued.
To answer your question directly...
- You can do this so long as the person has not claimed copyright or other anti-infringement claims.
- You do not plagiarise and otherwise steal any of the content the other party(s) have made.
- You do not create the exact same thing.
- You do not include anything that you do not permission for, such as titling a character "Superman" as it is a copyrighted term and I believe a trademarked character name.
However, to take the title from your thread "based on true story", if you're talking about writing a book, making a film or anything other than that having it based on a true story, as long as the story isn't copyrighted I'm fairly certain that you are allowed to make an adaptation of that story, however you can generally not entirely copy the story. There'd be no point in doing that, anyway...
If you were able to provide what exactly you were talking about (for example, an idea for a film) to put this in perspective. You can always visit government websites for plenty more information because all of these laws tend to work on county-wide regulations/laws.
