Episode two of The Original Series really raises the ante. It takes an idea that’s been used a few times in different media (the all powerful young child), but uses it in a stronger way than most other attempts.
What really makes this episode is Shatner’s acting. The episode hinges off his relationship with Charlie X. The scene with kirk explaining to Charlie why sexual harassment is a good example: a comedic beat, but the sheer awkwardness that Kirk is feeling is very clear.
There are also a bunch of firsts: first appearance of the infamous Jeffries Tubes, the first exploding panel, the first appearance of 3D chess. There are also some discordant moments compared to later Trek, such as the rec room scene where Uhura sings and Spock appears to smile.
Red Shirt deaths: none, technically. Everyone is returned at the end of the episode.
2 down, 735 to go. And 4 behind already.