Yesteryear (TAS episode):

Spock returns from a time-traveling research project he has been conducting with the use of the Guardian of Forever. When he emerges from the portal, he discovers no one onboard the Enterprise remembers him and that an Andorian officer, Commander Thelin has essentially replaced him.

History has recorded that Spock died at age 7 undergoing the Kahs-wan ordeal on Vulcan. Now he must go back through the time gate to his Vulcan childhood, and save the life of the child he was.

Trivia

According to the DVD text commentary for this episode, Los Angeles-area stations aired this episode first, instead of "Beyond the Farthest Star" (the actual premiere episode elsewhere) because of favortism allegations lodged by political opponents of George Takei, who was running for public office at the time. Instead of having to offer "equal time" to Takei's challengers, NBC stations in the L.A. area decided to air "Yesteryear" first, since it did not have any scenes with Hikaru Sulu (Takei's character) in it.^[1].

Canonicity

Some elements of this particular episode have made it into the canon over time. In fact, due perhaps to its solid storyline and the considerable information it gives about Spock's youth and Vulcan, "Yesteryear" has been considered canon by some of the production staff of Star Trek, and as such, information from this episode is more prevalent in later series than other animated episodes. Some specifics:

In the TNG episode "Unification", a dying Sarek describes details of Spock's childhood to Captain Picard. These details come directly from story elements of the animated episode.

The city of Shi'Kahr is referenced on a panel in "The Emissary" which read "Shi-Kar Desert Survival, Vulcan". The city was mentioned in "Fusion" in a reference to the Shi'Kahr Academy, and is apparently the origin of the name of the USS ShirKahr, which was mentioned in "Tears of the Prophets".

A display featured in "Eye of the Beholder" listed the Sepek Academic Scholarship; Sepek was the name of a Vulcan child in "Yesteryear".

Vulcan's Forge was referred to in "Change of Heart" and would be the focus of the Enterprise story arc in "The Forge", "Awakening", and "Kir'Shara".

Both the Lunaport and the kahs-wan ordeal are mentioned in "The Catwalk".

A wild sehlat was recreated with computer generated images CGI in "The Forge". T'Pol referred to the fact that Vulcan children often keep them as pets.

The nearby planet or moon seen briefly behind Shi'Kahr was shown in the original version of Star Trek: The Motion Picture.

Star Trek: The Animated Series

Season 1 (1973–1974)

"Beyond the Farthest Star" • "Yesteryear" • "One of Our Planets is Missing" • "The Lorelei Signal" • "More Tribbles, More Troubles" • "The Survivor" • "The Infinite Vulcan" • "The Magicks of Megas-tu" • "Once Upon a Planet" • "Mudd's Passion" • "The Terratin Incident" • "The Time Trap" • "The Ambergris Element" • "The Slaver Weapon" • "The Eye of the Beholder" • "The Jihad "

Season 2 (1974)

"The Pirates of Orion" • "Bem" • "The Practical Joker" • "Albatross" • "How Sharper Than a Serpent's Tooth" • "The Counter-Clock Incident"

Star Trek Vulcan stories

Star Trek

Balance of Terror • Amok Time • Journey to Babel • The Immunity Syndrome • The Enterprise Incident • All Our Yesterdays • Star Trek: The Motion Picture • Star Trek II: The Wrath of Khan • Star Trek III: The Search for Spock • Star Trek IV: The Voyage Home • Star Trek V: The Final Frontier • Star Trek VI: The Undiscovered Country

Star Trek: The Animated Series

Yesteryear • The Infinite Vulcan

Star Trek: The Next Generation

Sarek • Data's Day • Unification • Gambit • Star Trek: First Contact

Star Trek: Deep Space Nine

Take Me Out to the Holosuite • Field of Fire

Star Trek: Voyager