Captain Kirk and the crew of the USS Enterprise journeyed to the frigid planet Exo-III to look for the exobiologist Dr. Roger Korby who happened to be the betrothed of Dr. McCoy's temporary aide, Nurse Christine Chapel, who joined the Enterprise to find Korby.

Kirk and Chapel beamed down to a cave entrance per Korby's request. However, he wasn't there to greet them. Feeling something was amiss, Kirk asked two security officers to beam down from the ship - one keeping an eye at the entrance and the other joining Kirk and Chapel. The threesome proceeded to explore a system of caves. As they passed a bottomless abyss, the security officer suddenly vanished.

After a short while, they bump into Korby's assistant Dr. Brown. Chapel acknowledges him but is astonished that the man has no recollection of her. Brown guarantees them that the security guard had an accident, and the three proceed onward. Kirk communicates with the leftover security officer and instructs him to get in touch with the Enterprise for backup. Yet, a peculiar creature slaughters the officer before he has the opportunity.
Meanwhile, Kirk, Chapel, and Brown locate Korby, who discloses that an ancient race deserted the caverns. Korby demonstrates to Kirk and Chapel a machine that produces androids. With the support of Ruk, a still-functioning android from the period of the original inhabitants, Korby has made more androids, one being a wonderful lady he calls "Andrea." Brown is also an android. It is likewise uncovered that Ruk had murdered the security officers.

Korby develops a robotic replica of Kirk while Chapel is there, then implants Kirk's personality. As the real Kirk pictures himself making a derogatory comment about Spock being a "half-breed," Korby has the android copy of Kirk transmitted to the Enterprise with directions to find a planet that can be used for creating more androids. Unfortunately, when Spock queries the Kirk-android's instructions, it repeats the same insult that the honest Kirk had used. Understanding this differs from the faithful Kirk, Spock forms a security team to accompany the Kirk-android back down to Exo-III. When it reaches the planet, the Kirk-android meets Andrea, who destroys it when it refuses to kiss her.

The genuine Kirk can show Ruk that Korby is a peril to his being. Ruk starts to remember the fight between the "Old Ones" and the robots that caused his people's downfall centuries ago. Korby shows up, and Ruk challenges him. However, Korby terminates Ruk with a phaser. Right after that, during a battle with Kirk, the skin of Korby's hand is ripped off, uncovering that he is also a mechanical humanoid.

On the brink of death due to frostbite, it has been uncovered that Korby had moved his consciousness into a robotic body. He pleads for Chapel to accept that he is still the same person, yet Chapel is repulsed by the fact that he has done this to himself. Andrea, understanding that she loves Korby, kisses him, and in anguish, Korby fires the weapon that Andrea had in her possession, killing both of them.

Spock appears with the security team. However, the danger has already passed. When Spock asks about the whereabouts of Dr. Korby, Kirk answers, "Dr. Korby was never here." Chapel decides to stay on the Enterprise and conclude her mission. Spock explains to Kirk his disapproval of the phrase "half-breed" to alert him of the issue. The captain guarantees he will recall this if he finds himself in a "comparable situation."