This bonus featurette for a Doctor Who DVD manages to be quite a high quality
science documentary in its own right, rivaling shows like
"Cosmos", or the film
"What the Bleep Do We Know?"
Paul Parsons is particularly good at getting the concepts for each of the
four basic levels (types) of alternate universes across quickly and simply, such that
they remain clear and distinct in the minds of the audience long afterwards.
Stephen Baxter also does a good job of explaining how Level 3 alternate universes
resolve most time "paradoxes", aided by some simple but effective graphics of
a man walking down multiple paths.
The visuals for this featurette include a combination of actual astronomical
photographs and many beautiful, ethereal full-motion computer animations.
In the end, this is a great primer for the parallel universe topic,
and a highly-recommended place to start before digging deeper into the
subject, as one can do with longer documentaries like
"The Universe: Parallel Universes".