Sometimes it's helpful to ask one's brain direct questions. So today I said to my brain, "Hey, brain, why can't I figure out how this story begins? I'd really like to start writing. What's the problem here?"
To which my brain replied, "Well, Karina, you have completely forgotten to figure out half of the religion that is the core of the story, more than half of the political-system-in-turmoil, and pretty much everything about your main antagonist. I'd say that's something of a stumbling block, wouldn't you?"
There was a moment of internal shock as I came to terms with this. For a while the best I could muster was "Oh."
My brain's right, of course. Knowing the story part of the story does not mean that I'm ready to start writing. Silly me.