This is somewhat easier if the notion is backstory, or better yet background, to the original story. One reason can be that the backstory/background did not have many serial numbers in the first place. This can be because the writer properly kept it in its place. When a hero explains that the root of his powers is a diabolic rite that his father performed, with him in a central role, the details of how he was lured there, and what he felt, and the consequences thereof, may not matter to the story -- that the other writer told. Matters to yours if you transplant it and make it the foreground.
Or, on the other hand, the writer might not have developed it. Like stealing a political situation: a man who holds a high position; a woman who holds considerable power as his wife -- or widow -- hates him, and is willing to revoke his decrees believing him to be dead; another woman who's a rebel. And realizing that none of this has any substance to it. Why did the wife hate him? Why did he marry her? What is the motive for the rebellion? I could steal his men's loyalty as well, but that need not show that he treated anyone else well, or even justly. It was just three factions to enliven the lives of characters passing through.
To be sure, that does mean that stealing it is merely a matter of grafting on motives.