A transporter malfunction at the planet Alfa 177 in 2266, caused by magnetic ore dust, which divided Captain Kirk into two beings and stranded a landing party in lethal cold.
A geological technician named Fisher beamed up from Alfa 177 with his uniform covered in a yellow ore. The contamination affected the transporter, and when Kirk followed, he materialised twice: once as a gentle, indecisive man and once as a violent, appetite-driven one.
The Original Series episode The Enemy Within follows the search for the savage duplicate, who assaults Yeoman Rand. Spock and McCoy realise that the captain needs both halves to command. With the transporter unsafe, Sulu and the landing party are left on the surface as the night temperature falls far below freezing.
Scott and Spock repair the unit, testing it on a similarly divided local animal, which dies. Kirk takes the risk and is restored, and the party is recovered alive.