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.