Multiple level menu – how do I show only (relevant) part of the menu?

Apart from the blog-part of my site, my site consists of a number of pages.

The theme I’m using (and have modified quite a bit) shows the menu something like this:

Menu item A
– Menu item A.1
– Menu item A.2
— Menu item A.2.2
Menu item B
– Menu item B.1
– Menu item B.2
Menu item C
– Menu item C.1
— Menu item C.1.1
– Menu item C.2

and so on – I hope you can follow this.

Now, I want to modify my menu, so only the relevant part of the menu is show.

So, when looking at the front page, the menu should look like this:

Menu item A
Menu item B
Menu item C

with the ‘sub menus’ hidden.

If someone clicks ‘Menu item C’ the menu should change like this:

Menu item A
Menu item B
Menu item C
– Menu item C.1
– Menu item C.2

and if the user then clicks ‘Menu item C.1’ the menu should look like this:

Menu item A
Menu item B
Menu item C
– Menu item C.1
— Menu item C.1.1
– Menu item C.2

OK?

Is there a tutorial somewhere that can show me how to do this? Or a theme I can look at, that does this?

Thanks :o)