PPP2 - Carnival of Monsters 2

How I love Robert Holmes. There is a fantastic exchange early in this episode that is clearly an in joke (in an episode full of in jokes) that could be used to explain why most of the aliens in Doctor Who (and, let's face it, most of science fiction in general) are humanoid.

VORG (speaking of the humans in the Miniscope): Scientists have been amazed at the remarkable similarity between these little chaps and our own dominant life form.
ORUM: The resemblance is unpleasant.
VORG: ...Some scientists think that their discover refutes Valdeck's theory that life in the universe is infinitely variable.

Vorg's second line is another gem from Holmes. By mentioning Valdeck, of whom we have never heard, and his theory, which sounds as controversial as Darwin's theory was in the 1880s, Holmes instantly, with one throwaway line of dialogue, creates so much detail in fleshing out the world in which not only this story takes place, but the Doctor Who universe as a whole and how small (literally, in this episode) Earthlings are in the grand scheme of things.

The TV allegory is tamed down a bit in this episode, allowing The Doctor and Jo to become involved in both the action happening on the SS Bernice, as well as their exploration of the inner workings of the Miniscope. It also lets them encounter Barry Letts's favourite monster, the Drashig, at the end of the episode. The fact that the word "drashig" is an anagram of "dish rag", which is probably what Robert Holmes assumed the monster would be made out of (he wasn't half wrong), is yet another cheeky element to an already naughty little story.