The Enterprise tried to cross the energy barrier at the galaxy's edge, which gave helmsman Gary Mitchell dangerous psychic powers and forced Kirk to kill his friend.
Early in Kirk's command, the Enterprise followed the trail of the long-lost SS Valiant to the edge of the galaxy and attempted to pass through the galactic barrier. The ship was badly damaged, several crew members died, and two officers, Lieutenant Commander Gary Mitchell and psychiatrist Dr. Elizabeth Dehner, developed rapidly growing telepathic and telekinetic abilities.
Spock warned that Mitchell would soon regard the crew as insects. Kirk diverted to the automated lithium-cracking station on Delta Vega to maroon him, but Mitchell escaped. Dehner turned against Mitchell and weakened him before dying, and Kirk killed his old friend in a rockslide.