Lt. John Farrell
Jim Goodwin

Geological
Technician Fisher
Edward Madden

Transporter
Technician Wilson
Garland Thompson

From 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.