Doctor Who and the Silurians
The best classic Who serials tend to be the ones with great stories, or those that don't skimp on the monster action. This is something of a combination. We get our dated 70s alien treat with the very well realised reptillian Silurians (as well as a larger dinosaur-esque creature that looks a bit silly), but there's also a plot here that delves into some serious issues.
The big problem facing the third Doctor is whether or not he can convince the humans and Silurians (who have already been here for millions of years, hibernating underground) to co-exist in harmony. Naturally, things go awry on both sides, with a power struggle below ground resulting in a deadly disease being unleashed on mankind; while above the surface, the Brigadier shows his darker, militaristic side and decides to take drastic action.
And as you might expect, it doesn't end too well.
