He's a very well known theorist on Story - in fact he wrote the book "Story." If you're a writer, you owe yourself to read it. But I liked this one simplification he made of "story" after saying it can't be summed up simply.
"But let me try to answer that question by making a simple and clear definition of story itself. Story begins when an event, either by human decision or accident in the universe, radically upsets the balance of forces in the protagonist's life, arousing in that character the need to restore the balance of life. To do so, that character will conceive of what is known as an "Object of Desire," that which they feel they need to put life back into balance. They will then go off into their world, into themselves, in the various dimensions of their existence, seeking that Object of Desire, trying to restore the balance of life, and they will struggle against forces of antagonism that will come from their own inner natures as human beings, their relationships with other human beings, their personal and/or social life, and the physical environment itself. They may or may not achieve that Object of Desire; they may or may not finally be able to restore their life to a satisfying balance. That, in the simplest possible way, defines the elements of story - an event that throws life out of balance, the need and desire to restore the balance, and the Object of Desire the character conceives of consciously or unconsciously that they can pursue against the forces of antagonism from all of the levels of their life that they may or may not achieve."
-Robert McKee (from http://www.storylink.com/article/321 )
If that sounded interesting at all, you owe it to yourself to spend an hour listening to him talk about these things:
Play the movie.