"What Are Little Girls Made Of?" is episode seven of the first season of the science fiction television series, Star Trek. It was first broadcast October 20, 1966, and repeated, two months later, on December 22, the first episode of the series to be repeated on NBC. It was written by Robert Bloch and directed by James Goldstone. The title of the episode is taken from the fourth line of the 19th century nursery rhyme, "What Are Little Boys Made Of?."

In the plot, Nurse Chapel searches for her long lost fiancé, and uncovers his secret plan for galactic conquest.

On stardate 2712.4, the Federation starship USS Enterprise, under the command of Captain James T. Kirk, travels to the icy planet of Exo III to search for the exobiologist Dr. Roger Korby (played by Michael Strong), known as the "Louis Pasteur of archaeological medicine." Korby is the fiancé of Dr. McCoy's temporary assistant, Nurse Christine Chapel who has been searching for her missing lover for quite some time and signed on to the Enterprise for just this reason.

At Korby's request, only Kirk and Chapel beam down. When Korby is not there to meet them, Kirk has two security guards, Matthews and Rayburn, beamed down, but they are quickly disposed of by a large humanoid-looking figure in the caves: Matthews is pushed into a bottomless pit and Rayburn is suffocated. Kirk and Chapel find the doctor living in an underground complex of caves, left by an extinct race who once lived on Exo III. He refers to them only as "The Old Ones". Korby shows Kirk and Chapel machinery which creates androids. With the help of Ruk, a still functioning android left behind since the days of the Old Ones and the one who killed the two security guards, Korby has created more androids, one being a lovely woman he calls "Andrea".

...
Wikipedia