Some years before this second pilot, the S.S. Valiant had encountered a unknown energy barrier at the rim of the galaxy. Something had then made its captain destroy his ship.
The U.S.S. Enterprise finds the Valiant's disaster record-marker, which reveals that just prior to its destruction the crew had been searching library tapes for any information on psionics.
The U.S.S. Enterprise arrives at the edge of the galaxy and the same force affects the ship's drives that must have affected those of the Valiant. Kirk notices a strong personality change in his good friend, Lieutenant Gary Mitchell, whose latent psionic abilities are heightened by the force field. Soon Mitchell has gained enormous extrasensory powers, displaying many talents which include moving objects with his mind and controlling his own heart rate. To a smaller degree, Dr. Elizabeth Dehner is also affected by the force field's power and the two are drawn together by their common powers.
As Mitchell's powers grow, he becomes more dangerous. Spock believes that when the Valiant experienced the same phenomena, it destroyed itself to prevent the power from taking over the galaxy. Gary Mitchell confirms his fears when he informs them that he is becoming a god who will rule the humans. Spock feels that Mitchell's death is the only solution, but Kirk is unable to kill his old friend. Instead, he exiles him to an uninhabited planet. However, once on the planet's surface, Mitchell kills his guard and escapes, taking Dr. Dehner with him. Kirk follows with a phaser rifle and Mitchell attempts to kill Kirk with his psionic powers.
Witnessing this display, Dr. Dehner realizes how inhuman and dangerous Mitchell has become and tries to help Kirk defeat him. Without remorse, Mitchell kills Elizabeth Dehner. Before he can refocus his psi powers, Kirk creates a rockslide that entombs Mitchell, killing him.
Afterwards, Spock admits to Kirk that for the first time in his life, he felt something akin to human emotion.