After the Enterprise encounters a barrier around the galaxy, Gary Mitchell and Elizabeth Dehner begin developing extraordinary mental abilities. With great power comes great insanity, however, and Kirk is forced to kill Mitchell, with the aid of Dehner, who sacrifices herself to weaken Mitchell.

Notes

Gary's transformation is too sudden–he goes from a normal person, and a close friend of Kirk, to a power-crazed madman in the space of a few minutes of screen time.

Similar message to the previous episode, "Charlie X", that absolute power corrupts absolutely. Trek explores the theme of the effects of power repeatedly. Wesley has his day, Riker has his, and there's the whole thing with Q junior in Voyager. Even "The Man Trap" touched on this.

This episode has the characters somewhat different than usual: Sulu is a physicist and the ship's doctor is Mark Piper rather than Leonard McCoy.