Before it even developed its core audience of kindred spirits, Star Trek created this bottle episode about a socially awkward teenager who finds it difficult to cope with life among humans - and who has godlike powers he can't help but use when he feels threatened or disrespected. It's an adolescent story from an adult perspective, with Charlie representing many young men who yearn to be liked and understood with the science fiction twist throwing him into a power struggle with Captain Kirk.
For William Shatner, it's a rare chance to portray a father figure, and he does it with such ease it makes you wonder why it took 44 more years before he landed the part of a sitcom dad. His Kirk balances patience and respect with firmness and conviction. In short, he gives the character a disarming kindness while still staying true to his duties as captain. But it's Robert Walker as the titular Charlie, who carries the episode with his brilliant and convincing performance, giving the character an innocent enthusiasm that's lovable and pitiable at the same time. 26 year old Parker sees to it that 17 year old Charlie X wears his heart on his sleeve, and the compelling performance creates a heartbreaking character study that carries through from the first to last act. By the end, the dues ex machina solution (repeated many times throughout the series) feels earned because the study has run its course.
But make no mistake, "Charlie X" is still Star Trek trying to find its way. The rhythm and joy the writers would find in subsequent episodes isn't here yet. In fact, like "The Man Trap" this one's really a downer, though Grace Lee Whitney (who turns in a fine performance as Yeoman Rand, the object of Charlie's first crush) has talked about spying Walker at a roller rink in 1979 and skating up to him with a, "Hi Charlie!" So I'd like to think that somewhere in an alternate universe, Charlie and his crush are able to find peace together skating around in circles.
The episode (which takes place on Thanksgiving) is notable for Gene Roddenberry's first and last Star Trek cameo: he's the voice of the galley chef telling Captain Kirk about the meatloaf turning into turkeys.
Remastered Edition: With all the action happening aboard the ship, most of the new effects are the standard Enterprise establishing shots. One notable exception is the cargo vessel Kirk references at the top of the episode. It's not seen in the original version (which gets all its space footage from the two pilots), but in the remastered edition it appears with a design that's been borrowed from the freighters seen in the Animated Series episode "More Tribbles, More Troubles" (where they appear as robot ships)... a nice touch of retroactive continuity.