"Shhhh, young me, don't try to make sense of all this."
While investigating the past through the Guardian of Forever, Spock's past is accidentally changed so that he no longer exists as the Enterprise's first officer, and now he must go back to his own past to fix whatever happened in the past that changed everything.
Written by Original Series veteran D.C. Fontana, "Yesteryear" is notable for being the one episode of the Animated Series that Gene Roddenberry is said to have considered canon.
This episode provides examples of
- An Aesop: A very blatant one in favor of animal euthanasia, outright stating that it is "logical" to end a wounded animal's suffering rather than sustain its life only for it to live in agony. Fontana commented that it was her proudest moment in the making of TAS... and the one that met with the most controversy from viewers.
- Continuity Snarl: Either the animator did not get the memo that Vulcan has no moon, or he just plain ignored it.
- Friendless Background: This episode lets us see Spock having this. His agemates torment him endlessly for being "a Terran" and Sarek, who expects his son to act like a Vulcan, is disappointed by Spock reacting to their teasing.
- Inciting Incident: Spock comes back through the Guardian a couple of steps behind Kirk and Erickson, and thus erases himself from history.
- Grandfather Paradox: Spock in this episode saves his own life.
- Kids Are Cruel: We get our first glimpse at Spock's childhood in this episode... and it's not pretty.
- My Future Self and Me: Spock guides his seven-year-old self through a dangerous Vulcan rite of passage.
- Never Say "Die": Actually averted. The computer explicitly states that Sarek's son died in childhood, and D.C. Fontana included the aspect of putting I-Chaya to sleep specifically to teach children that pets die and it's best to let them pass with dignity.
- One-Liner, Name... One-Liner: The last line of the episode.Spock: "Times change, doctor. Times change."
- Portal to the Past: The Guardian of Forever is used to do historical research, but things go awry, setting up the main plot.
- Ret-Gone: Spock in this episode — rather he did exist but died in childhood.
- Ripple-Effect-Proof Memory: Only Spock and Kirk remember the original timeline.
- Rite of Passage: We learn of another Vulcan ritual, this time the kahs-wan, a survival test of adulthood for Vulcan youth. The basis for the kahs-wan was to survive ten days without food, water, or weapons in Vulcan's Forge.
- Sequel Episode: The episode brings back the Guardian of Forever from "The City on the Edge of Forever", which causes time to change, and allows Spock to restore the timeline. It also follows up on elements of Spock's childhood referred to by his mother, Amanda in "Journey to Babel".
- Shoot the Dog: I-Chaya is stung by a venomous animal. The vet gives young Spock the option of saving him but forcing him to live with constant pain or euthanizing him. Spock chooses the latter.
- Stable Time Loop: Spock mentions the "cousin" who helped him survive the desert. There is a difference, however — in his present memories, his pet sehlat did not die.
- Stock Sound Effect: The le-matya's sounds are the iconic roar of Godzilla. It's rather amusing that his roar was used here, but Toho wouldn't let Hanna-Barbera use it in their own Godzilla series a few years later.
- Temporal Paradox: If Spock hadn't survived to adulthood, he couldn't have protected his younger self from an early death.
- The Time Traveller's Dilemma: We certainly hope Thelin the Andorian got a comparably good job in the rewritten timeline. At least he accepted his fate bravely, and Spock wished him a long and prosperous life in whatever circumstances he would be placed in. (Like most Trek time travel, it's unclear whether or not the version of him serving aboard Kirk's Enterprise got to keep doing that in a divergent timeline.)
