TOS #7: What Are Little Girls Made Of?

I think this is the first episode in which undergroud survivial caves are mentioned, in yet another connection to ST2. Also, this episode has a barren ice-world -- just like ST6.

I love, in this episode, how Kirk trusts Dr. Corby only until he beams down and doesn't see him, at which point he immediately disregards the Doctor's request and calls for backup. He is, of course, correct to be suspicious, because this dude is lurking:

He's a neo-sapien android with Asimov robot programming. Also a precursor to Data, his name is "Ruk."

This is definitely the hottest girl so far in the series. And her name is... Andrea! Sadly, she is also an android.