Ok, ich suche was kontextfrei verständliches raus.
Usually I come up with a plot idea. Then I evaluate this idea, look at it from different angles, including the most important: Would I like to play this plot?
Then, I set up the plot, using the w's: where, when, who, why. WHY is the most important question! If you cannot answer the WHY, your plot usually gets very thin and tends to be illogical.
If you come up with a good why, this why tends to need some story before or beside the main plot. This is how the plot idea evolves into a storyline.
Other details are added while writing the plot.
@NSC: mea culpa ... NSC is in fact a german abbreviation for NichtSpielerCharaktere, in English you call them NPC for Non Player Characters.
@Ordering: This may be useful, but normally story chapters are separated, so separating the story into chapters reduces the information load for a location in this chapter.
Remember: If you close a chapter, there is no going back. Information, Equipment and Plot can (and should!) be carried to the next chapter, but you cannot go back. So Information on what can be done in the location in a previous chapter is an "over-information", which will slow down the retrieval of relevant information.
Just think of all your chapters as separate games finished at the chapter end. So you write multiple design documents in one.