Dr. Roger Korby
Michael Strong

Andrea
Sherry Jackson

Ruk
Ted Cassidy

Dr. Brown
Harry Basch

Matthews
Vince Deadrick

Rayburn
Budd Albright

The Enterprise enters orbit around the planet Exo III to investigate the whereabouts of Dr. Roger Korby who has not been heard from for five years. Christine Chapel is particularly interested in the outcome as she still maintains the hope of being reunited with Korby, who is her fiance. They are pleasantly surprised to receive a transmission from him on arrival, as the chances of his survival had seemed small.

Kirk and Christine Chapel beam down to an arranged location in the middle of a vast maze of tunnels beneath the planet's surface where they are met by Dr. Korby's assistant, Dr. Brown. Although well known to each other in the past, Chapel is taken aback by Brown's coolness towards her, seeming to be strangely aloof. They are taken to Korby, who is accompanied by a beautiful female called Andrea. Korby wants to secretly show Kirk the discovery he has made but Kirk insists on contacting his ship to advise the situation. He is prevented from doing so and, in the ensuing struggle with Brown, Kirk injures him, revealing him to be an android.

The captain is then subdued by another android, a giant called Ruk. It is explained to Kirk that both Ruk and Andrea are also androids, left behind on the planet by The Old Ones, who originally inhabited it. Korby has discovered a machine for creating androids into which minds can be created or transplanted and has plans to create 'the perfect society' by transferring everyone's consciousness into near-immortal android bodies.

James Kirk refuses to have anything to do with this so Korby uses the machine to create a duplicate 'Kirk' which he hopes to use to take control of the Enterprise and allow the fulfilment of his ideals. The android captain is 'allowed' to beam back to the Enterprise but his identity is questioned by Spock when he insults the Vulcan officer, an idea that was placed in the android's mind by Kirk during its creation.

In the events that follow, both the giant Ruk and the 'Kirk' android are destroyed, climaxing with the discovery that Roger Korby is himself an android, already having undergone the 'treatment'. Unable to face the dissolution of his plans and confronted with the realisation of his real nature, it ends when Korby holds the Andrea creation close to him and disintegrates them both with a phaser blast.