a landing party gathering specimens on the planet Alfa 177, James Kirk
beams back to the Enterprise but arrives feeling dizzy and weak. The transporter
is checked out using an alien animal but it arrives back on board as two
creatures, one docile and the other savage and aggressive. The transporter
is shut down for tests, leaving the landing party to fend for themselves
as night falls and the temperature starts to drop dramatically.
It is some time before the increasingly strange behaviour of 'Captain
Kirk', not least of which is the attempted seduction of Yeoman Janice
Rand, leads Spock to suspect what has happened and he eventually corners
and reveals the existence of Kirk's violent doppelganger. The two Kirk
personalities confront one another and the violent one immediately tries
to kill the other. They are separated with a warning from Spock that death
to one would probably result in both of them dying.
The transporter repairs are completed and the two alien creatures are
beamed down to the planet and back again. They return as a single animal
but the shock of the transition has killed it. This leaves Kirk and the
command crew with a difficult decision. When it becomes apparent that
the mild Kirk is progressively losing his powers of decision and command,
they are forced to try and reunite them.
The beam back proves to be successful as Kirk's intelligence allows him
to understand what is happening to him whereas the animal did not and
apparently died of fright. The landing party is beamed back from the surface
of the planet just in time to save them from freezing to death on the
planet's harsh surface.