More wisdom from Mr Wendig on the elements of story:
The throughline is an invisible thread that binds your story together. It comprises those elements that are critical to the very heart of your tale — these elements needn’t be the same for every story you tell but should remain the same throughout a given story. You don’t switch horses in midstream, after all. Because that’s just silly. You have a horse. You’re in the middle of a stream. That horse over there, you can’t trust him. He might be a total dick. Plus, if you leave your current horse, you’ll hurt his horsey feelings. Do you want that on your conscience? Can you handle seeing your ex-horse try to drown himself in the very stream you just crossed on another mount? You bastard.