A frozen world whose vanished inhabitants built android technology beneath its surface, where Dr. Roger Korby takes refuge in What Are Little Girls Made Of?
Exo III is a planet whose surface became frozen after its sun began to fade, driving its ancient inhabitants, known as the Old Ones, into underground caverns. There they built sophisticated androids, which eventually turned on their makers and destroyed them, leaving only the machine Ruk behind.
In What Are Little Girls Made Of? the Enterprise searches for the exobiologist Roger Korby, who has been missing on Exo III for five years. Christine Chapel, his fiancée, joins the search and learns that Korby has used the Old Ones' equipment to build android copies of people, and that he may now be one himself.