How can I create a menu that has an overlay upon click?

I have 5 items listed within a 200x200 box, and would like for the details of the clicked item to appear as either an overlay that spans only that box or as a div that completely replaces the original 5 options. Kinda like an accordion menu, but with the nav menu items completely disappearing rather than just moving out of the way. To return to the original layout, I'd like a simple [x] as the close button.

I've searched everywhere for a tutorial, so I'm guessing maybe it's either not possible or a really bad idea. If the latter, let me know your thoughts as to why. Thanks a ton.