[[caption-width-right:350:Hey, Spock? Wasn't your [[MateOrDie pon farr]] ''last'' season?]]
The PowerTrio beams down to the planet Sarpeidon, intent on evacuating the denizens before the sun goes supernova. The only person they find is a librarian called Mr. Atoz and his [[MesACrowd clone army.]] Guess the library just wasn't getting any volunteer workers. Mr. Atoz tells Kirk and co. that everyone has gone already. Gone where? "Wherever they wanted to go." Mr. Atoz invites Kirk and co. to peruse his library, containing images of Sarpeidon's history. [[ReadingIsCoolAesop Because even if the sun's going supernova, there's still time to read!]] Or at least look at moving pictures. Kirk looks at images from a pre-industrial age. Bones and Spock look at images from the planet's ice age. Kirk hears a woman's scream and jumps to investigate. "No, wait!" shouts Mr. Atoz. "You haven't been prepared!" After a psychedelic light show, Kirk disappears and reappears in something right out of ''Literature/TheThreeMusketeers''. Spock and Bones ignore Mr. Atoz's warnings and jump in after Kirk, only to find themselves on the cliff of an icy mountain.

Now, all they have to do is go back through the portal. Yeah, not as easy as all that....
!!Tropes for this episode include:
* AbsenteeActor: Sulu, Chekov and Uhura don't appear. Scotty does not appear on screen but has several voice-over lines. In no other episode are only three regular members of the crew seen in person.
* TheAloner: Zarabeth has been stranded in the Ice Age by herself for who knows how long, as a tyrant's punishment for the misdeeds of her family.
* AncientKeeper: The librarian Mr. Atoz. He offered to help the Enterprise landing party go back in time the way the rest of the planetary population had.
* BadassGrandpa: The librarian manages to fight off captain Kirk quite handily.
* BittersweetEnding: Poor Zarabeth. Poor ''Spock.''
* BurnTheWitch: Kirk gets accused of witchcraft when the voices of Bones and Spock are taken to be that of spirits he's conjuring. Fortunately, the judge is someone who was sent back in time to this era.
* CoolGate: The Atavachron, a gate to the past.
* DidNotGetTheGirl: Spock has to leave Zarabeth because it's the only way for Bones to return and she can't follow.
* ExactTimeToFailure: The Enterprise can calculate the supernova explosion to the second.
* HumanAliens: While this episode is hardly unique in using this trope, it's striking how not only do the Sarpeidon natives look exactly like humans, they went through a historical period ''exactly'' like 17th-century Europe.
* IWillOnlySlowYouDown: Bones collapses from exhaustion after wandering the frozen wasteland with Spock and tells Spock to leave him. Spock throws logic to the wind and picks Bones up and helps him walk.
* LiteraryAllusionTitle: From Shakespeare's ''Theatre/{{Macbeth}}'': "And all our yesterdays have lighted fools the way of dusty death...."
* MeaningfulName: What better name for a librarian than "A to Z"?
* NoGoodDeedGoesUnpunished: Kirk hears a woman screaming and rushes to assist, and winds up trapped centuries in the past, accused of theft and witchcraft.
* NoOneGetsLeftBehind: Spock, of all people, does this when [=McCoy=] collapses from exposure to the elements and tells the Vulcan to go on without him.
* NoOSHACompliance: The Atavachron's PortalDoor looks just like a door going outside. It's also smart enough to know which disc you were viewing to select your destination, but not smart enough to know if you were "prepared".
* NubileSavage: Spock and Bones meet one of these. It turns out she's from the planet's future, and was exiled to the distant past via a time machine by a dictator.
* OffscreenTeleportation: In the teaser, the dramatic moment leading into the opening credits is Mr. Atoz apparently doing this, always turning up ahead of the away team as it moves around the library. It's subsequently explained that he has a couple of duplicates of himself, created by some unexplained process, that help him run the library.
* OOCIsSeriousBusiness: Spock starts out getting visibly frustrated, escalating into murderous rage. He kisses a girl passionately. [[ArsonMurderAndJaywalking And he eats meat.]] It seems that all Vulcans have a loose psychic link with one another[[note]]Continuity! In "The Immunity Syndrome" this is how Spock senses the destruction of the ''Intrepid'', though he doesn't pick up details. In a line in "Amok Time" that got cut for time, he says Vulcans always know their proximity to their homeworld.[[/note]], and since they are thousands of years in the past and all Vulcans are in a savage state....,
* OutOfCharacterMoment: The original script Spock falling in love and kissing Zarabeth but averted in the filmed episode at Creator/LeonardNimoy's insistence. Instead, his behavior was {{hand wave}}d with the explanation that, having gone back in time, he became like the savage Vulcans of that time.
* OutrunTheFireball: The ''Enterprise'' outruns a supernova at the end of the episode.
* PortalDoor: the Atavachron, a machine that creates a portal door/wall to a time in that planet's past.
* RedheadInGreen: The woman Kirk saves from a Musketeer. She even has hints of an [[{{Oireland}} Oirish]] accent.
* SamusIsAGirl: The [[StarWars Jawa]] with gigantism that saves Spock and Bones from freezing to death pulls back the hood to reveal a young Mariette Hartley!
* SciFiWritersHaveNoSenseOfScale: Spock tells Zarabeth he comes from a planet "millions of light years away". The Milky Way Galaxy is only about 100,000 light years across. So no.
* StockFootage: The imagery our heroes watch include stock footage of a horse drawn carriage and a frozen wasteland.
* SwordFight: Kirk gets in one with a Musketeer. He wins! He can sword fight with Musketeers, he can hold his own against [[Recap/StarTrekS1E18Arena the Gorn]], but he can't take on [[BadassGrandpa an 80 year old librarian?]]
* TemporalSickness: As the inhabitants of the planet are sent back to their past to escape their star going nova, they go through a preparatory treatment ahead of time. Kirk, Spock and Bones travel without being given the treatment, and will die unless they can [[IncrediblyLamePun get back in time.]]
* TwirlOfLove: Spock's kiss with Zarabeth turns into this.
* YouCantGoHomeAgain: Zarabeth tries to convince Spock and Bones this. The judge tries to convince Kirk this. Operative word: "tries".
** For Zarabeth and the judge, at least, it is entirely true.