The USS Enterprise meets the Antares to take charge of Charlie Evans sole survivor of a transport ship that crashed on the planet Thasus. For 14 years, Charlie grew up there alone, stranded in the wreckage, learning how to talk from the ship's computer systems which remained intact.
He is to be transported to his nearest relatives on the colony Alpha V. Crew members of the Antares speak praises about Charlie, but seem pleased to see the boy removed from their ship. After the transfer, they bid the Enterprise an unusually hasty goodbye and depart. Charlie undergoes a medical examination by Dr. McCoy. He tells the doctor the crew of the Antares did not like him very much, and that all he wants is for people to like him.
When the Antares gets nearly out of sensor range, it transmits a warning message to the Enterprise, but the message gets cut off before it can give the warning. Shortly after, Spock determines that the Antares has blown up.
Charlie quickly becomes obnoxious and shows signs that he possesses strange powers. First, he develops an infatuation with Yeoman Janice Rand. He presents her with a bottle of perfume, which turns out to be her favorite scent. Having observed a man in engineering seal an agreement to go to the recreation room with a slap on the rear, he does the same to Rand.
Charlie meets Rand later in the rec room, where Mr. Spock plays a Vulcan lyrette and Lt. Uhura sings. Charlie is annoyed with being a subject in Uhura's performance as well as with Rand paying more attention to the song than to him, so he causes Uhura to temporarily lose her voice.
In an attempt to get Charlie interested in a woman his own age, Rand introduces him to Yeoman Tina Lawton, but Charlie only has eyes for Rand and brushes her off. Later, Kirk tries to teach the young man how to fight. Sam, Kirk's training partner, laughs at one of Charlie's falls, and Charlie makes him vanish before Kirk's eyes. Shocked, Kirk calls for security guards to escort Charlie to his quarters, but Charlie says he will not let them hurt him; he then makes their phasers disappear. Charlie admits he used his powers to destroy the Antares, but says the ship would have blown up on its own sooner or later and insists, "They weren't nice to me."
Charlie discovers Kirk's plans to divert from Alpha V, and takes control of the Enterprise. He forces Spock to recite Earth poetry, turns Tina into a lizard, and chases down Rand. When she resists his advances, he makes her disappear. Charlie then goes on a rampage, hideously transforming or vanishing crew members at will.
Meanwhile, a Thasian ship approaches the Enterprise. The Thasian commander appears on the bridge, saying that his race gave Charlie his powers to help him to survive on their world, but these powers render him too dangerous to live among humans. The Thasians return Yeoman Rand and repair the damage Charlie has done, apart from the Antares. They promise to take Charlie to live with them. Charlie begs Kirk for forgiveness and pleads with him to not let the aliens have him, that they don't feel anything. Despite Kirk's statement that Charlie belongs with his own kind, the aliens take him.