Once I have a story in my head, I can't rest properly. Not until it is out on the page or fixed down in some other way. That is the situation I am in right now.
I can't let you know what it is just yet. But the ideas are coming all in a rush and I need to find a process for imposing a structure on them.
The structure I choose initially is the Three Act Structure. I want to pick out two of the major plot points and fix them down. The first will come approximately a quarter of the way into the story. The second will come more or less three quarters of the way through. Then - with those in place - I'll be able to start putting the various other plot devices into place within that structure. It seems very likely that some of the raw ideas won't have a place and will need to be thrown out. But at least I will know.
Does it sound as if I am artificially imposing a predetermined structure? Yes, I am. But that is the magic of story craft. There is infinite possible variation - even within such rigidity.