Stardate 1312.4: The Enterprise encounters a record-marker left behind by the S.S. Valiant years ago as it tried to go through an energy barrier at the rim of the galaxy. After reading the badly damaged tapes they learn that the captain of the Valiant became obsessed with information on human psionics just before he ordered the destruction of his vessel. Despite the mystery, the Enterprise continues on to the rim of the galaxy in its own effort to get through the barrier. Once there the same energy forces which turned the Valiant away damage the Enterprise's engines and cause a strange behavioral change in Kirk's close friend, Gary Mitchell, changes which begin to alarm the command officers, especially Spock who watches closely as Mitchell gains incredible extrasensory powers. As Mitchell's powers grow stronger he begins to feel less connected with humanity and more concerned with his god-like abilities. Soon, he begins to see humans as insignificant; things to be squashed like insects. Despite Kirk's reluctance, he and Spock manage to contain Mitchell long enough to get to Delta Vega, a mining planet, and transport him down to the surface. As repairs proceed with the Enterprise, Mitchell's powers grow beyond Kirk's abilities to control. By now, Dr. Dehner realizes that she too has been affected by the energy barrier and begins to gain powers similar to Mitchell's. Mitchell escapes with Dehner, but Kirk follows with a phaser rifle. When he finds them, Mitchell tries to kill Kirk with his powers. Dehner, now realizing how corrupt Mitchell has become, helps Kirk weaken Mitchell before she succumbs to his psionic powers. But before Mitchell can fully regain his strength Kirk creates a rockfall and entombs Mitchell, bringing the threat he represents to humanity to an end.