This isn't the one I mentioned. In the one I referenced, The Sontaran lands in ancient Egypt and gets the locals to start building a Sontaran base. One night some locals hear him communicating with the the home world about what will really happen. They trap the Sonataran in the kings tomb. Flash to the present and a British scientist is opening the kings tomb which will free the Sontaran (who is still alive somehow). Turns out the ancients always feared (kinda like "The Mummy") this and crush the Sontaran as he exits with a huge square stone. :)