Stardate 1312.4: The Enterprise is approaching the galactic barrier when they pick up a transmission from a long-lost space probe. Kirk is playing chess with Spock as he waits to intercept the probe. They find it and beam it aboard, and discover it's an old-style ship recorder, badly damaged. Kirk puts the ship on alert and heads for the bridge. En route they meet with helmsman Gary Mitchell, and they all arrive on the bridge. All department heads arrive, along with Dr. Elizabeth Dehner, who has been assigned to watch the crew under stress.
Spock deciphers the signal and reports that the tapes are burned but apparently the ship, the Valiant encountered some unknown force and was thrown back from the galactic barrier. The log shows the crew asked for all records on ESP and then the captain ordered self-destruct. Kirk orders them to continue and they approach the galactic barrier. The ship begins to take heavy damage from the strain and both Dehner and Mitchell are enveloped in burst of energy. They manage to escape the barrier after losing nine crewmen, although Dehner and Mitchell are alive. Jim checks on Mitchell, whose eyes are now glowing silver.
The ship's space warp is badly damaged and Dehner reports that the dead crewmen had sections of their brain burned out. Spock concludes that those with high ESP ratings were the ones targeted, and Mitchell and Dehner have the highest ratings. Kirk goes to visit Mitchell, who seems to be normal although he has a renewed interest in reading. Mitchell wants to return to duty and when Kirk refuses, Mitchell's voice echoes briefly. Kirk returns to the bridge where Spock reveals that Mitchell is reading inhumanly fast and going through the entire ship's library.
Later, Dehner talks to Mitchell and he reveals he can control his body's autonomic functions. He temporarily shuts his body down entirely and then revives himself. He also has complete recall of everything he's read. Navigator Lee Kelso comes in and Mitchell warns him that he's overlooked some burned out power points. Kelso checks it out and discovers that Mitchell was right. Scotty reports that Mitchell tested his telekinetic control of the engine controls and Dehner is forced to admit it's possible Mitchell has vast powers. Sulu predicts that Mitchell's powers are growing geometrically. Spock advises that they go to a nearby automated dilithium cracking station to refuel, and strand Mitchell there. The only other alternative is to kill Mitchell, but Kirk orders them to set course for the station on Delta Vega.
Kirk, Spock, and Dehner go to see Mitchell who boasts of his new abilities and displays that he can read minds and knows what they have planned. Mitchell easily throws the two of them back with his powers and refuses to go down, as he plans for the ship. Kirk manages to knock him down long enough for Dehner to sedate him and they beam down to the planet. They put Mitchell in a force field brig and Kirk orders Kelso to prepare a self-destruct switch.
Mitchell regains consciousness and tries to appeal to Kirk's friendship. When that fails he throws himself at the force field and his eyes lose their silver glaze. He seems human again, but quickly reverts to his godlike self.
Scotty completes repairs and notes he sent a phaser rifle down to Spock. Spock warns that they'll be lucky to escape and Kelso completes the self-destruct. Kirk orders Kelso to activate the self-destruct if Mitchell escapes.
Kirk and the others watch Mitchell, unaware that he uses a cable to garrote Kelso before he ca do anything. Dehner still claims that Mitchell isn't evil, but Mitchell eradicates the force field and stuns Kirk and Spock. He takes Dehner to a mirror and show that her eyes are now silvered as well.
Dr. Piper wakes up Kirk, who orders him not to revive Spock while he goes after Mitchell and Dehner. Kirk orders them to return to the ship and leave in 12 hours if they haven't heard from him, and have the planet subjected to lethal radiation.
In the wilderness, Mitchell brings the barren landscape to life and boasts that she'll soon be able to do the same. Kirk makes his way across the terrain with the phaser rifle and Mitchell easily senses him and sends Dehner to greet him so she can see how important they are. Kirk tries to get her to help him against Mitchell, predicting that he'll go insane with power. Mitchell arrives and Kirk tries to shoot him without effect. Mitchell telekinetically flings the phaser rifle away then creates a grave and a boulder to bury Kirk with. Kirk notes for Dehner that Mitchell is still driven by human frailty but the would-be god forces him to pray to him. Dehner turns against Mitchell, launching energy bolts at him. The two exchange blasts and both are knocked down and revert to human.
Kirk launches himself at Mitchell trying to knock him out. Mitchell manages to fight back until Kirk tries to bash his head in but hesitates for just a moment. Mitchell recovers his power and Kirk is forced to drop back until Mitchell easily drops him and picks up a huge boulder. Kirk knocks him into the grave and grabs the phaser, bringing down the boulder on him and crushing him to death. The dying Dehner apologizes before expiring.
Back aboard the ship, Kirk gives commendations to both of them and notes that neither of them asked for what happened to them. Even Spock admits that he felt for them.