About the only 2 bits of prep I do for most games is a handful of major NPCs and a flowchart. the flow chart will have scenes I plan out say a fight I plan at a bar but there will be an alternate line so if the players don't follwo the clue to the bar what happens instead. Now I will drag the pcs into the plot. A pc that doesn't get involved will become involved. This won't happen by forcing them into x or z location but I might hit them with an encounter related tot eh plot and try to feed themself to get on the right track. If they really find somethign else do do I will dump what I ahd but go on with what they want but the other plot will complete and in a CoC game that might mean New York gets destroyed by a monster or in a Cyberpunk game some replicant clone imposter gets to be President. This might have future repurcsions on the players.
The point is I do have a story its the backdrop to the game the players don't have to get invoilved in it but I rather they did and I will manipulate them ruthlessly to make sure they do.