Charlie Evans
Robert Walker Jr.

Nellis
Dallas Mitchell

Navigator
Don Eitner

The Thasian
Abraham Sofaer

Yeoman Tina Lawton
Patricia McNulty

Captain Ramart
Charles J. Stewart

Crewman
John Bellah

Crewman
Garland Thompson

Enterprise arranges a rendezvous with the cargo ship Antares in order to take on board Charlie Evans who, as a three-year old child, was the sole survivor of a crash landing on the planet Thasus and who had only recently been rescued. On his arrival aboard the Enterprise he becomes enamoured with Janice Rand who is the first girl that he has ever seen. Charlie, who is unused to human company and the social graces, roams the Enterprise making errors and becomes annoyed by his own inability to fit in with the crew.

He decides to seek some 'fatherly' advice from an extremely uncomfortable Captain James T. Kirk. Now under way again, the Enterprise receives a transmission from the Antares that ceases abruptly when the cargo vessel explodes. It becomes increasingly obvious that Charlie has some strange mental abilities and was, in some way, responsible for the destruction of the Antares because 'they didn't like him'.

When a crewmember laughs at Charlie's awkwardness at Judo the boy causes him to 'disappear'. Aware of the size of the potential problem facing the Enterprise, Kirk tries unsuccessfully to confine him. Following this, he tries to steer the ship away from the highly populated colony of Alpha V, which had been their destination, in order to protect the inhabitants.

This really annoys Charlie who uses his powers to assume control of the Enterprise. After Janice Rand refuses his advances, he also makes her disappear and almost does the same to Kirk and Spock before they manage to convince him that he needs their help to keep the Enterprise going. In order to strain Charlie's powers and levels of concentration to the limit, Kirk and Spock set about activating every mechanism and system that they can find. Charlie realises what is happening but, before he can carry out reprisals, a message is received from a ship purporting to be Thasian in origin.

It turns out that after his crash on Thasus, the indigenous aliens, an incorporeal race, gave Charlie his abilities in order to help him to survive. They realise that he is now too dangerous to be allowed to return among ordinary humans so, despite Charlie's imploring that he doesn't want to go, they take him back to Thasus after returning Janice and the other crew members from the limbo to which they had been sent. They could not, however, undo the fatal events that had overtaken the Antares and her crew.