10 Greatest Sequels In Star Trek
6. Yesteryear
Yesteryear is a crucial episode for audiences to understand exactly who and what Spock is. It is a sequel to The City On The Edge Of Forever, even if it merely the presence of the Guardian of Forever that links the two. That said, not every sequel must pick up, beat for beat, on every aspect of the original.
Yesteryear takes the idea of a sentient time machine - also known as Carl, though that would come later - and gives the audience a chance to explore Spock's childhood. It doesn't de-fang him, nor retcon any established lore. Mark Lenard returns as Sarek, further deepening the connection to The Original Series.
While The Animated Series tends to vary in audience estimation, the stories - Yesteryear especially - are among Star Trek's best. This episode manages, in twenty minutes, to show us what's happening with Carl, how Spock became who he is, and what an Aurelian actually looks like.