The way this game is played is that each person posting posts one sentence which carries the story chronologically in reverse order.

Boy goes through door.
Boy opens door.
Boy approaches door.

... is an example of reverse chronology.  To give us something to work at, we'll set a goal for 100 posts.  The beginning of the story should be met by the 100th post.  (If you think that's too short, we can adjust it).

The two people shot out of the mountain.
And they all lived happily ever after.
THE END