Star Trek: The Original Series has a lot of great episodes, running the gamut from the emotionally devastating ("The City on the Edge of Forever") to the comedic ("The Trouble With Tribbles"). And yet, for my money, no episode is a better example of everything the Original Series got right than "Journey to Babel". Image: CBS via Trek Core
Some great Trek episodes, like "The City on the Edge of Forever", don't necessarily need to be Star Trek in order to be good. Certainly, they are enriched by taking place in an established universe with characters we already know and sympathise with, but the core of the story could be told anywhere. Everything in "Journey to Babel", however -- regardless of the characterisation, the drama, the world-building and the humour -- could only belong to Star Trek.
The plot revolves around a bunch of dignitaries being shuttled by the Enterprise to a meeting to vote on a planet's entry to the Federation; the Vulcan ambassador and his wife happen to be Spock's parents. Meanwhile, there's a mystery vessel following the Enterprise.
The first plot point means getting a better look at Federation politics, which includes introducing the Andorians and the Tellarites. It's strongly implied that the Tellarites are against the planet's admission because they can only exploit its resources if it's not protected by Starfleet. It's such a small thing, but it reminds us that what we see on the Enterprise isn't going on everywhere in the universe, and that the Federation is a coalition of many planets and not just a home for humans. (Later instalments... forgot this.)
The second element of the episode, Spock's parents, focuses on the personal. Spock's Vulcan father Sarek has a heart problem that flares up while he's on the Enterprise. He needs Spock's blood to get the operation, but, for some reason, Spock's contribution to this surgery would require him to be put under. This wouldn't normally be a problem, but someone just tried to kill Kirk and he's incapacitated; given the delicate situation, Spock doesn't see the logic in saving one man and endangering the mission. (Everyone acts like putting Scotty in charge would doom everyone. Possible, but still, harsh.)
The scenes with Spock's parents are perfect Trek. Jane Wyatt and Mark Lenard deserve so much praise for their portrayals of human Amanda and Sarek the Vulcan. The tension between Sarek and Spock is evident, even though they aren't technically acting emotionally. And Amanda's tearful recounting of her half-Vulcan son being bullied as a child is beautifully done, as is her slap after Spock chooses to take command of the Enterprise instead of saving his endangered father.
After I made this gif, my soul ascended to heaven, having finally glimpsed perfection.
It took all of J.J. Abrams' first Star Trek movie to do with Spock's character what this episode accomplished in just a few scenes. You understand implicitly that Spock is very much like his father, even though he made decisions that Sarek really, truly felt were wrong.
The third bit, the mystery ship, gives us Kirk in all his glory. Nothing is more Kirk than him pretending to be healthy enough to command the ship so that his friend can save his father's life. That's how the plot with Sarek needing surgery gets resolved: Kirk intends to fake health just long enough for Spock to go to sickbay. It gets extra Kirk-y when the mystery ship attacks just as he hauls his nearly collapsing arse into the captain's chair.
The final act of "Journey to Babel" should be shown to anyone trying to write Kirk, Spock and McCoy. Even in bad shape, Kirk manages to save the day. Spock figures out a plot to sabotage the vote right as he's about to be sedated. And McCoy just wants everyone to stop running around and talking so they can heal.
My version of heaven is this scene, on a loop. And it doesn't even include to the part earlier in the episode where Amanda embarrasses Spock by telling his friends about his childhood pet.
If D.C. Fontana had just done this episode, she'd have made an invaluable contribution. But she was also the story editor on the series, rewrote episodes and wrote her own. While being a young woman working in the '60s. You can tell from this episode just how invaluable her understanding of this universe was. While there are episodes of the Original Series that are better examples of the science fiction allegory the show became famous for, this is the best example of why Trek itself, as a universe, is worth watching.