In the episode, the Enterprise picks up an unstable 17-year-old boy who spent 14 years alone on a deserted planet and lacks the training and restraint to handle his superhuman mental powers wisely.

The USS Enterprise meets the merchant vessel Antares to take charge of Charlie Evans, the sole survivor of a transport ship that crashed on the planet Thasus.

For fourteen years, 17-year-old Charlie grew up there alone, stranded in the wreckage, learning how to talk from the ship's computer systems, which remained intact.

Despite his eagerness to please, Charlie becomes obnoxious since his lack of upbringing has left him with no knowledge of social norms or control of his emotions.

Charlie meets Rand in the recreation room, where Mr. Spock plays a Vulcan lyrette and Lt. Uhura suddenly starts singing.

However, the Thasians reject Kirk's argument that Charlie belongs with his own kind, and with a final echoing wail of "I wanna stay!"

The premise for this episode formed part of Gene Roddenberry's original March 1964 pitch for Star Trek, under the name "The Day Charlie Became God".

"[2] Gene Roddenberry made an uncredited audio cameo as the cook (or mess officer) who exclaims that the turkey-shaped meatloaf in the galley ovens has turned into real turkeys.

[6] In 2024 Hollywood.com ranked Charlie X at number 13 out of the 79 original series episodes, calling it "bold, primary-colored fantasia of ‘60s pop art".