"What Are Little Girls Made Of?" is a first season episode of Star Trek: The Original Series. It is episode #7, production #10, and was first broadcast October 20, 1966. It was repeated two months later, on December 22, 1966, and was 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?".
Overview: Nurse Chapel searches for her long lost fiancé, and uncovers his secret plan for galactic conquest.
On stardate 2712.4, the 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. Korby is the fiancé of Dr. McCoy's temporary assistant, Nurse Christine Chapel, and is known as the "Louis Pasteur of archaeological medicine". Chapel 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 also beamed down, but they are quickly disposed of - Matthews is pushed off a cliff 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 is used to create realistic androids. With the help of Ruk, a still functioning android left behind since the days of the Old Ones, Korby created more androids, one being a lovely woman he calls "Andrea". It turns out Ruk is the one who has killed the guards.
Chapel recognizes Korby's aide Dr. Brown, but is surprised the man does not remember her. In reality, Brown is also an android created as a prototype for Korby's diabolical plan that will replace key personnel in the Federation with android duplicates under his control.
Korby keeps Chapel at his side, but imprisons Captain Kirk and makes an exact android duplicate of him, so like Kirk himself that it can fool even Chapel. The Kirk android even knows Kirk has a brother named George Samuel Kirk, whom only he calls "Sam". Kirk managed to place a flaw in his duplicate. While Kirk's mental patterns were being duplicated in the android, the original Kirk repeatedly thought "Mind your own business, Spock. I'm sick of your half-breed interference! Do you hear?" This became imprinted in the android's personality matrix. (In truth, Kirk has only respect for his second officer, and would never express such an opinion.)
Korby has the duplicate Kirk beamed aboard the Enterprise with orders to go to Minas V to begin the spread of android duplicates throughout the galaxy. Korby is convinced the duplicate Kirk will fool the Enterprise crew, but Spock realizes something is wrong when the duplicate Kirk shouts the insulting words "half-breed" at Spock for questioning his orders. Spock then forms a security team and follows the duplicate Kirk back down to Exo III to investigate what he is up to.
Meanwhile, the real Kirk, while being guarded by Ruk, convinces the android that his master Korby is a threat to his continued existence and must be destroyed. Bestirred, Ruk remembers that the same kind of clash between the Old Ones and the androids led to his civilization's demise centuries ago, and gradually concludes that under present circumstances, conflict is again is inevitable. Korby enters and Ruk confronts him, but Korby destroys Ruk with a phaser. Shortly afterward, in a struggle with Kirk, Korby gets his hand caught in a door. When the skin tears back, it reveals to Chapel's horror that he is also an android.
Believing it to be the original, Andrea destroys the duplicate Kirk with a phaser when "he" refuses to kiss her.
It is now revealed that Dr. Korby, when dying from severe frostbite, had transferred his mind to an android body so that he might live on. However, Kirk convinces Korby that he is nothing more than a machine and has lost his humanity forever. Chapel is also repelled by what her fiancé has done to himself. Realizing she loves Korby, Andrea kisses him and in despair, Korby fires Andrea's phaser between the embracing pair, destroying them both.
Spock arrives with the security force, but finds that the crisis has passed, since Kirk and Chapel are now safe. When Spock inquires about Dr. Korby's whereabouts, Kirk replies: "Dr. Korby was never here." In the end, Chapel decides to stay on with the Enterprise and finish out her tour of duty.