An odd, yellowish ore with magnetic properties on a crewman's clothing affects the transporter, and when Kirk is subsequently beamed up, he is split into two "halves". One half of him possesses his benevolent qualities, but is indecisive. The other possesses his malevolent qualities, but is afraid. At first the crew is unaware of the problem, and Kirk's reputation suffers. Then they realize what has happened, and that the transporter is useless - just when they need to bring back the remaining planetary expedition before the sub-zero night kills them! And they must discover how to "reassemble" the captain when it becomes clear that neither half can survive for long without the other.