A lot of fans don't consider The Animated Series to be part of official canon, but then, a lot fans also acknowledge the significance of "Yesteryear" itself, the most routinely-praised episode of the series.
It's significant in a number of ways, actually. The least significant element is the return of the Guardian of Forever from "City on the Edge of Forever," which has apparently become a routine tool for studying history.
The episode is better known for Spock's journey to his own past. The framing story deals with altered history (due to his trip, Spock is actually erased from history, and he's replaced by an Andorian among the Enterprise crew), but that's just more window-dressing. This visit to Vulcan involves the return of his parents Sarek and Amanda, last seen in "Journey to Babel" and an examination of Spock's inner turmoil concerning his dual Vulcan-human lineage. It also features a Vulcan rite-of-passage ritual and Spock's pet! Established plenty that later ended up in other episodes, thereby solidifying it as part of the canon.
Lots of details. They're all good. It's the most fascinating episode of the series.
Star Trek (the 2009 reboot) even has conscious echoes of "Yesteryear," from a conversation Spock and Sarek have to the bullying the young Spock faces from his peers.
|via John Kenneth Muir|
By any definition a classic.
four quarter analysis
franchise * series * essential * character