A powerful teenage boy wreaks havoc aboard the Enterprise.
The cargo ship Antares docks with the Enterprise to deliver young Charlie Evans, the lone survivor of a crash on the planet Thasus 14 years earlier. The personnel of the Antares, Captain Ramart and Navigator Tom Nellis, seem unusually eager to leave—and when the Antares is destroyed, suspicion falls on Charlie.
Aboard the Enterprise, Charlie begins to work "miracles." At first these are harmless pranks, kept in check by Captain Kirk, whom Charlie comes to regard as a father figure, but when Yeoman Rand spurns his advances, she is erased from existence. Various other ship personnel are victimized, and in his eagerness to arrive at Colony Alpha V, Charlie assumes control of the Enterprise.
In an attempt to tax Charlie's control abilities, Kirk switches on the starship's interior systems to their fullest extent. Then a shimmering alien face materializes on the Enterprise bridge and identifies itself as a Thasian, come to take Charlie back. Despite Charlie's pleas, he vanishes along with the alien, doomed to spend the rest of his life alone on the bleak planet Thasus.