Master Cat's on main page, ONLY sub cat's on specific cat pages

I know the title is a little confusing, and that's why I'm posting a new thread question.

I can't figure out how to search for this, and there are too many other threads to read :wacko:

Maybe someone can answer, or, point me to the appropriate thread.

Here goes.

I want to have all of the Master categories on the Main (index) page menue only. When you click on a Master category, and go to that page, then, ONLY the sub-categories would be listed in the menue.

Or, perhaps, have all the Master categories always listed along the top of the page, and, when you click on one of those, the sub cats for that master category are listed in the menue sidebox, and, as long as you "stay" in that master category, the sub cats are always the only thing listed in the menue sidebox.

If you click on another Master Category, you are taken to that category, and voila, only the sub cats for that master cat are listed in the menue side box.

Basically, a sort of Mini Mall. The Master cats along the top would be the "Store" and the sub cats in the sidebox would be the Departments for that store and that store Only.

Is this possible? Am I making any sense? Have I completely lost my mind?:wacko: :wacko: :wacko: