Collapse / Fold SubPage Navigation

I’m having trouble getting my sidebar navigation to work how I want it to.

Page navigation is horizontal, in the header. Each page has sub pages, and many of those have their own sub pages. I want the side bar to only show the sub pages for each parent page, and if somebody clicks on a sub page with sub-sub pages, the list of sub-sub pages expands:

For example, your sidebar of a certain section looks like this:

* sub page 1
* sub page 2
* sub page 3

then you click on of those and get this:

* sub page 1
– sub sub page 1
– sub sub page 2
* sub page 2
* sub page 3

I’ve been looking into several options and plug-ins, but none are really working right. All the other questions i’ve found in the wordpress forums are generally unanswered and very old, as are most of the plug-ins i’ve seen. Collapsing Pages seems like it should work, but shows all the parent pages all the time.