At one time, David Morrell says, he was taught that there are five types of plots:
1) Human against Human. 2) Human against Nature. 3) Human against himself/herself. 4) Human against Society. 5) Human against God.
Morrell thinks there's only one, and it goes like this: Somebody wants something, and somebody else wants to stop them from getting it. This is conflict, which is at the core of plot. But for a story, Morrell says, you also have to have "why." You have to know the motivations of the antagonists.
Sounds pretty simple when you put it that way.