While approaching the edge of the Galaxy, The U.S.S. Enterprise detects a recorded distress signal from the S.S. Valiant, an Earth vessel missing for two centuries. The Enterprise finds a damaged ship recorder from the Valiant, with the last messages indicating something about extrasensory perception in humans, and the captain ordering the ship’s destruction.
When the Enterprise reaches the edge of the Galaxy, it encounters an energy barrier that damages the ship’s warp engines, and Lt. Commander Gary Mitchell begins to display increasing mental powers.
With no warp power, the Enterprise travels to a nearby lithium cracking station on Delta Vega, where Kirk intends to strand Mitchell before his mental abilities begin to endanger the crew. The ship’s new psychiatrist, Dr. Elizabeth Dehner, whose own mental abilities were enhanced by the energy barrier, helps to neutralize Mitchell’s powers, and Kirk is forced to kill him before Mitchell becomes unstoppable.