Need some help in advanced redirect...

I have a index.html main page. I need to write a script to redirect diffrent user page to pass through my index.html and then count down redirect back to user page.

1) User enter URL at address bar.....
then redirect user to another site like (index.html)..

2) The site (index.html) will countdown 20second and display countdown message and progress bar...

3) after countdown then redirect to the first URL entered by user...

but it have to work in dynamic method because every diffrent url entered at the address bar also need to pass to the index.html and redirect again to other diffrent site.