With only one or two exceptions, the lousiest episodes of the original STAR TREK were those where they were stuck on the ship. This often had the tendency to make the shows very static and slow compared to others and CHARLIE X is no exception.
Robert Walker, Jr. (who looked a lot like his ill-fated father) plays Charlie--a horny and misunderstood young man who is supposedly discovered alone on a planet--the lone survivor of an earlier expedition. Instead of having Charlie discovered by the Enterprise, I think they just had him beam over from another ship to save on expenses for filming the episode. Charlie seems too normal for a kid who mostly raised himself (like Mowgli). Oddness, though not immediately apparent, is indeed present. It seems that Charlie has psychic powers and soon begins to act like a spoiled baby with dark powers (a lot like Billy Mumy from the Twilight Zone series where he terrorized a small farming town with his mental abilities). Most of what Charlie does isn't all that scary (such as having someone turn into an iguana--and not even showing the transformation), though I really loved when he was royally ticked off and overheard people laughing--and made their faces disappear (you see one of these people--a woman grasping about the ship). However, what really makes this a bad episode is the ending where the ultra-powerful spirits who made Charlie the way he is came back to retrieve him. Despite having totally terrorized the crew and killed many of them, Kirk begs the spirits to let Charlie stay with the humans!!!!! No way does this make any sense and frankly, I would have loved to see Charlie get killed to end his reign of terror.