An example from my current Fate game: I adapted a D&D-like module, so it should be a great example!
A lamia was abducting people and hypnotizing them, making them participate in debauched rituals and then either using them as thralls or killing them. I had a solid timeline on when people had disappeared, a subplot with a wannabe-sorcerer who had found out the info and used it to blackmail a friend of the PCs, who was, at time of play start, just enthralled.
The module had set scenes:
1) meet father of friend and join the search
2) gather info from 3-4 locations
3) one of them gets kidnapped but escapes
4) hunt down blackmailer and handle him
5) (optional) go to one of the Lamia's parties
6) break into lamia's house and destroy her source of power
So I prepped all the main characters, giving them aspects and skills. Also the main areas and buildings. The module had its scripted version of past events and what would happen next, so that was done for me.
That's the prep part.
In play, the players made narrative changes to find a witness and also to entirely avoid scene #3. They skipped scene #5 completely. One of the narrative changes they used involved the creation of a London street gang that is now a major part fo the campaign.
So:
- The dungeon was stocked ahead of play
- The GM created very little in the moment
- There was a mapped plot, but it was defined as provisional
- Due to both narrative control and player actions, the mapped story changed
I pretty much so the same thing in CoC, Gumshoe, D&D, 13A Deadlands Classic and all the other systems I run -- the systems differentiate mostly in whether the provisional plots are changed mostly due to dice rolls, player agency, or narrative control.
Hope that helps!