Enterprise recovers the pitted and burned emergency buoy from the wreckage of Valiant which penetrated the odd energy barrier that surrounds the galaxy. Garbled recordings suggest odd consequences of that journey, and even that Valiant's own captain ordered her destruction. Enterprise also visits the barrier, and it has odd effects on certain members of the crew with high esper ratings. One in particular, Kirk's friend Gary Mitchell, appears to have developed nearly godlike mental and physical abilities, and lost his moral compass as well. Now Kirk must discover how to stop - and maybe kill - his old friend before that becomes impossible, and Mitchell goes on to conquer the Federation!
 

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.