The Enterprise meets with the survey ship Antares and picks up a passenger: a young boy named Charlie Evans, a castaway. The Antares crew are clearly uncomfortable around Charlie yet eager to praise him. Charlie keeps interrupting and Kirk gently reprimands him. As they leave, Yeoman Rand arrives and Charlie is clearly taken with her, once he figures out she's a girl.
Charlie was the sole survivor of a crash when he was 3 and managed to survive on his own for 14 years. McCoy gives him a medical exam and Charlie notes he fed himself on concentrates and other stuff he found, and learned English from the crashed ship's memory banks. Charlie says that the Antares crew didn't like him but McCoy assures him that he's okay. Charlie then approaches Rand and gives her a bottle of perfume, her favorite. She agrees to meet with him later and he slaps her on the bottom, just as McCoy did earlier, and she tries to tell him it was inappropriate and suggests he talk to Kirk or McCoy about it.
McCoy discusses Charlie's unlikely story with Spock, and wonders if the planet he came from, Thasus, may have been inhabited by the alleged never-seen Thasians. Kirk tries to get McCoy to take on Charlie's introduction to ship's crew.
In the rec room, Uhura and Spock sing and after Uhura gently mocks Charlie, he does something to cause her to lose her voice. He then demonstrates a series of remarkable card tricks to Rand, claiming someone on the Antares showed him. Later Charlie finds Kirk as he's telling the chef to make turkey for Thanksgiving, then asks him why slapping Rand on the butt was in appropriate. Kirk tries to explain and is interrupted with the Antares calls them. They're barely in range and the captain tries to warn them about something, but the signal is cut off. Spock determines that the Antares has been destroyed. They're interrupted when the chef calls up to report that the meat loaf he put in the oven has transformed into live turkeys.
Kirk plays 3-dimensional chess with Spock and admits he's concerned about the Antares' destruction. Spock is suspicious of Charlie who seemed to know about the survey ship's destruction just as it happened. Kirk wins the game and Charlie asks for a game with Spock. Charlie quickly loses and after Spock leaves, Charlie concentrates and melts the pieces with his mind. He leaves and Rand tries to introduce Charlie to a younger yeoman, Lawton, but he brusquely dismisses her and insists on talking with Rand. It becomes clear he has a crush on her and she goes to Kirk, who talks to Charlie. Charlie refuses to hear anything against his feelings for Rand and Kirk takes him down to the gymnasium to show him how to fight. Kirk demonstrates some throws with crewman Sam, then on Charlie who doesn't do well. When Charlie messes it up, Sam laughs and Charlie makes him "go away." Kirk has security try and take Charlie to his quarters but he throws them off and makes a phaser disappear. Kirk locks wills with Charlie and commands him to go to his quarters on his own.
After being informed every phaser on the ship has disappeared, Kirk meets with Spock, who confirms that the legendary Thasians have the same powers as Charlie. McCoy assures them Charlie is actually a human. They can't take him to their next destination, Colony 5, since he would cause widespread devastation in an open environment. Charles come in and admits he made a baffle plate on the Antares disappear, causing its destruction because they weren't "nice" to him.
Kirk tries to contact Colony 5 and change course, but the communication panel shorts out and the controls are locked. Charlie arrives on the bridge and forces Spock to recite poetry. Kirk gets him to back down and Charlie stalks off down the halls, transforming Lawton into a lizard. He stalks into Rand's quarters and she secretly launches the intercom, alerting the bridge. She refuses him and Kirk and Spock arrive, only to be thrown back. She slaps him and he makes her go away, then warns Kirk that he needs them… for now.
They try to lure Charlie into the brig but he makes the entire wall disappear and easily escapes. He ages one woman and silences a number of others when they laugh, causing one's face to seal over. He goes to the bridge where Uhura repeats they're receiving a signal. Charlie dismisses it and hints that he is responsible.
Later Kirk decides enough is enough and figures that Charlie has reached his limit, not having sent anyone away since he took control of the ship. Charlie barges on to the bridge and Kirk confronts him while McCoy and Spock turn on every ship's system. Charlie puts Kirk through increasing pain but is interrupted when the signal comes in again. Charlie cries out and a Thasian's image appears on the bridge. The Thasian says they were unaware Charlie had left and can't do anything for the Antares, but have restored everyone on the Enterprise back to normal. Despite Kirk's suggestion that they try and train Charlie so he can live with his own kind, the Thasian warns that Charlie would have no choice but to use his powers and would either destroy them, or they'd have to destroy him. The Thasian takes the pleading Charlie away while Rand quietly cries for the boy.