After the cancellation of The Original Series (TOS) in 1969, the live action show proved popular in syndication and generated significant fan enthusiasm. This resulted in Roddenberry's decision to continue the series in animated form. Much of the original cast returned to provide voice-overs for their characters. Show writers David Gerrold and D. C. Fontana characterized The Animated Series as effectively a fourth season of The Original Series (which would effectively make the second season of TAS the fifth season of TOS.) After the conclusion of The Animated Series, the adventures of the characters continued in live-action theatrical films, the first being the 1979 film Star Trek: The Motion Picture.
Season 1 (1973–74)
1 "Beyond the Farthest Star"
While exploring on the outermost rim of the galaxy the USS Enterprise is pulled into the orbit of a dead star. Trapped there, the crew discovers that there is an ancient derelict pod ship trapped with them as well.
2 "Yesteryear"
Spock must travel to the past to rescue his younger self from danger.
3 "One of Our Planets Is Missing"
The Enterprise encounters a giant cloud creature that feeds on the energy of the planets that lie in its path. They determine it is heading for Mantilles, home to a Federation colony governed by former Starfleet officer Bob Wesley (featured in the TOS episode "The Ultimate Computer)".
4 "The Lorelei Signal"
Investigating a sector of space where starships have disappeared every 27 years, the Enterprise finds a race of beautiful women living on the planet Taurus II.
5 "More Tribbles, More Troubles"
While the USS Enterprise escorts two robot cargo ships carrying quintotriticale, a new seed grain, to famine-stricken Sherman's Planet, it encounters a Klingon battlecruiser pursuing a Federation scout ship. When the Enterprise rescues the pilot, the Klingons attack with a new energy weapon and demand that the pilot be handed over to them.
6 "The Survivor"
Patrolling near the Romulan Neutral Zone, the USS Enterprise finds a ship carrying Carter Winston, a Federation citizen and philanthropist who has been missing for five years.
7 "The Infinite Vulcan"
While visiting the newly discovered planet Phylos, Lieutenant Sulu picks up a walking plant, called a Retlaw, and is poisoned. The alien species that inhabit the planet, who are plantlike beings, approach and save Sulu's life, but kidnap Mr. Spock.
8 "The Magicks of Megas-tu"
While exploring near the center of the galaxy, the USS Enterprise is caught inside an energy/matter vortex and all her computer systems fail. A being named Lucien appears on the bridge, repairs the ship's systems and takes the crew to explore his planet, Megas-Tu.
9 "Once Upon a Planet"
The Enterprise crew revisits the "amusement park" planet first seen in the Classic Trek episode "Shore Leave" hoping for some rest and relaxation.
10 "Mudd's Passion"
The USS Enterprise receives orders to arrest Federation outlaw Harry Mudd, who is accused of selling fake love crystals. Intercepting Harry on the mining colony of Motherlode, they bring him aboard the Enterprise.
11 "The Terratin Incident"
While observing a burnt-out supernova, the USS Enterprise picks up a strange message transmitted in a two-hundred-year-old code.
12 "The Time Trap"
While exploring the Delta Triangle, where many starships have disappeared, the USS Enterprise is attacked by several Klingon vessels. During the battle, they are caught in an ion storm. The Enterprise and one Klingon battlecruiser are drawn into a spacetime vortex and end up in a timeless dimension.
13 "The Ambergris Element"
While exploring the water planet Argo, Captain Kirk and Mr. Spock are transformed into water breathers by the planet's undersea inhabitants, the Aquans. In order to return to their normal selves, they must enlist the help of the Aquans to capture a giant sur-snake, whose venom holds the antidote.
14 "The Slaver Weapon" ~ Adapted from the short story "The Soft Weapon" by Larry Niven
In the shuttlecraft Copernicus, Mr. Spock, Uhura and Sulu are en route to Starbase 25 to deliver a stasis box—a rare artifact of the Slaver culture when the Kzinti intervene.
15 "The Eye of the Beholder"
The disappearance of a scientific team lures the USS Enterprise to investigate near Lactra VII. The starship Ariel is located there, abandoned, with its captain having transported to the planet's surface.
16 "The Jihad"
The USS Enterprise arrives at the Vedala asteroid, where Captain Kirk and Mr. Spock have been summoned to learn about a stolen religious artifact, the "Soul of the Skorr", whose theft could ignite a galactic holy war.
Season 2 (1974)
17 "The Pirates of Orion"
Spock contracts a fatal illness, and the cure can only be found with dangerous Orion pirates.
18 "Bem"
The Enterprise crew is taken captive by a race of primitives on a newly discovered planet.
19 "The Practical Joker"
A strange energy field causes the Enterprise computer to play practical jokes on the crew, but the humor soon turns to danger.
20 "Albatross"
Doctor McCoy is arrested for allegedly causing a deadly plague which once ravaged the planet Dramia.
21 "How Sharper Than a Serpent's Tooth"
A mysterious being threatens to destroy the Enterprise if the crew is unable to solve an ancient puzzle.
22 "The Counter-Clock Incident"
An unusual spaceship pulls the Enterprise into a "negative universe" where time seems to flow backwards.
- In the original Star Trek series, the main character was given the name James T. Kirk. It was not until the animated series that writer David Gerrold expanded on the "T", establishing Kirk's middle name as Tiberius. By coincidence, on Gene Roddenberry's first series The Lieutenant, the principal character was William Tiberius Rice. According to Gerrold, he had been influenced by I, Claudius, and had approached Roddenberry with his choice of middle name, who agreed; Gerrold did not learn about the earlier use of the name until 2014.
- The animated series introduced a three-armed, three-legged alien member of the bridge crew with a long neck named Arex and a cat-like alien crew member named M'Ress. According to Roddenberry, budget limitations would have made it impossible for either alien species to appear in a live action series of the time.
- The USS Enterprise in this series, while supposedly the same ship as from the original series, had a holodeck similar to the one later seen in Star Trek: The Next Generation, which was set about eighty years later. It only appeared once, in Chuck Menville's "The Practical Joker", and was known as the "Rec Room". This feature was originally proposed for the original series but was never used.
- A personal force field technology known as the life support belt was seen only in Star Trek: The Animated Series. In addition to supplying the wearer with the appropriate atmosphere and environmental protection, it permitted the animators to simply draw the belt and yellow glow around the existing characters, instead of having to redraw them with an environmental suit. A version of the life support belt later appeared in an early Star Trek: The Next Generation novel, The Peacekeepers, where they were referred to as "field-effect suits".
- An anti-pollution public service announcement was created for non-profit Keep America Beautiful featuring the ST: TAS characters and original cast voices. In the ad, the Enterprise encounters the "Rhombian Pollution Belt".
- At the end of the first season of Star Trek: The Next Generation, all licenses for Star Trek spin-off fiction were renegotiated, and the animated series was essentially "decanonized" by Gene Roddenberry's office.
- In the Star Trek: Deep Space Nine episode "Once More Unto the Breach", Kor referred to his ship, the Klothos, which was first named in the TAS episode "The Time Trap". Other DS9 episodes to make reference to the animated series include "Broken Link", where Elim Garak mentions Edosian orchids (Arex is an Edosian) and "Tears of the Prophets" where a Miranda-class starship is called the USS ShirKahr (sic) after ShiKahr, the city from "Yesteryear". In the episode "Prophet Motive" the title of healer is resurrected from "Yesteryear" as well. Vulcan's Forge is also mentioned in "Change of Heart", in which Worf wants to honeymoon there with Jadzia Dax, as well as in episodes "The Forge", "Awakening" and "Kir'Shara" from Star Trek: Enterprise.
- The Star Trek: Enterprise episodes "The Catwalk" and "The Forge" included references to "Yesteryear", the latter featuring a CGI rendition of a wild sehlat. The remastered Original Series episode "Amok Time" featured ShiKahr in the background as Spock beams up at the episode's ending and the remastered version of "The Ultimate Computer" replaced the Botany Bay-style Woden with an automated grain carrier from "More Tribbles, More Troubles".
- The 2009 film Star Trek also references "Yesteryear", featuring a nearly identical scene in which a young Spock is confronted by several other Vulcan children, who bully and provoke him for being part human.
- The 2017 series Star Trek: Discovery episode "Context Is for Kings" has Spock's foster sister Michael Burnham state that their mother Amanda read Alice in Wonderland to them as children, as in the episode "Once Upon a Planet."
- The 2021 series Star Trek: Lower Decks episode "I, Excretus" features a Pandronian drill administrator named Shari yn Yem, the first Pandronian in the history of Trek to appear outside of TAS. The race was introduced in TAS episode "Bem".