Play video in fullscreen before entering page

I’m trying to add a video that plays before you get to the homepage. Either it lies on a toned background so you can see the rest of the page and after it finishes it enters automatically OR it could play above the header and when it’s finished it scrolls down so that the header is at the top of the screen. If it’s the seconds option I don’t want the video to start on every page, only the homepage and only when you enter the page for the first time.

I’m using the Twenty Twelve theme with a child theme.

Help please!