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The starship Enterprise has a rendezvous with the tiny survey ship Antares to pick up a special passenger, Charlie Evans (Robert Walker Jr.). The sole survivor of a spaceship crash on the remote planet Thesus when he was three years old, Charlie spent 14 years alone, learning to talk and how to survive from the crashed ship's computer tapes. The Enterprise is to transport him to his closest living relatives, and he is eager, after so many years alone, to meet more people. Science officer Spock (Leonard Nimoy) is doubtful of the boy's story, because of the sheer barren nature of Thesus, but Chief Medical Officer Dr. McCoy (DeForest Kelley) is willing to take him at face value, and Captain Kirk (William Shatner) finds the boy looking up to him as a guide into this new world. Charlie is like any healthy 17-year-old in that he wants to be liked, and wants friends and more; he also develops a hopeless crush on Yoeman Janice Rand (Grace Lee Whitney). But Kirk (William Shatner) and company discover that Charlie is unlike any 17-year-old, in that he has somehow developed immense and terrifying mental powers, apparently from his years on Thesus, enabling him to transform matter at will, and that he has an easily roused temper and an easily pricked ego that can bring those powers to bear in an instant, destroying the Antares as her captain was trying to warn Kirk about his passenger and making people who displease him disappear, or transforming them, sometimes horribly, as when he closes up the eyes and mouths of a group of laughing crew members. Kirk, Spock, and Dr. McCoy try to defeat him by overtaxing his powers once he takes control of the ship, but it is only when a ship carrying actual Thesians -- a race supposedly only existing in legend -- makes rendezvous with the Enterprise that Charlie's power is broken and his crew restored whole. But now Kirk must face a terrible choice about what to do with Charlie: risk the safety of others or send him away with the immaterial, wraith-like Thesians, consigned to a life of perpetual loneliness and solitude forever.