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!