grade 5/5
One of the fledgling series most compelling and poignant early stories. Robert Walker carries the show with a masterful performance as Charlie, combining the vulnerability and the menace that the role calls for. Walker often was cast in "troubled youth" roles. It is obvious that this epsiode, as noted before, was at least partly inspired by the Twilight Zone episode "It's a Good Life", whose original author, Jerome Bixby, later went on to become a Star Trek writer himself.
Also as noted before, the episode does a great job of using the minimal existing sets to create the illusion of a massive spacegoing vessel with many decks full of bustling crewmembers. Scenes such as the rec room and the gymnasium segements, full of off-duty crewmembers, help to achieve this effect, as do the many corridor shots full of people coming and going.
Excellent early offering, a good dramatic episode, with a not entirely happy ending, showing the dramatic potential of this new series. The negative critical reaction to "The Man Trap" when it first premiered is well-documented . . . but I wonder what the critics reaction to this superior follow-up was.
Incidentally, "Charlie X" was the very first Trek epsiode I ever saw, while still a young child. It got me hooked at an early age.