The best stories are those that can be summed up in one sentence.
Which is to say, a solid story hinges upon its Situation.
The Situation is not the whole story but the essence of that story’s conflict, which will in turn shape the events of your plot.
Here is an example of a situation:
On a certain summer evening, a charismatic priest, no longer young but not yet old, who has devoted his entire life to God, watches the last particles of his faith dematerialize before his eyes, when a mysterious woman in the confession booth convinces him that God is a logical impossibility.
This situation comprises the essentials of a good story because it carries with it the essence of the conflict.
Conflicts come from characters in the face of adversity.
Conflict is at the heart of every good story because conflict is what will hold the reader’s interest.
A conflict means that there is a clash, and that there are obstacles to overcome. These two things are what hold one’s interest.
Holding the interest of the reader — or, in the case of movies, the viewer — is the total goal.
If, for example, you’re thirsty and you look in your refrigerator and find only orange juice and soda pop, there is here a certain conflict that you must resolve: i.e. you must choose between those two things.
Obviously, though, this isn’t the kind of conflict upon which a story can be built. But why?
The answer is that the choices aren’t important enough to be of lasting interest.
To be of lasting interest, the conflict must be of certain importance.
Of course, not everyone regards the same things as important — and that is one of several reasons that there are genres: romance, western, history, mystery, horror, fantasy, and so on.
In all genres, though, the basic principle remains the same: there must be a clash of values (which is to say, a conflict), the clash must be relevant, and the best stories can state the essence of that conflict in one sentence.
That one sentence is The Situation.