Exposure to the galactic barrier gives Gary Mitchell godlike abilities, forcing Kirk to confront his oldest friend.
The Enterprise crosses the barrier at the edge of the galaxy, and Kirk's close friend Gary Mitchell begins to develop silver eyes and growing psychic powers. As his abilities increase, so does his contempt for ordinary humans, and Kirk must decide whether to stop him.
This was the second pilot, and it sold the series. It features Scotty and Sulu early, and the line about going where no man has gone gave the franchise its famous phrase. The story anticipates many later Trek tales about godlike power corrupting those who gain it.