History Recap / StarTrekS3E23AllOurYesterdays

* YouCantGoHomeAgain: Zarabeth tries to convince Spock and Bones this. The judge tries to convince Kirk of this. Operative word: "tries".
** For Zarabeth and the judge, at least, it is entirely true. The Atavachron both transports one through time and "prepares" them for it, altering their bodies to survive in the chosen timeframe (how and why is handwaved). If you've been "prepared" for a past time, returning to the future kills you. If you ''haven't'', you can only survive a few hours in the past.

* ArtisticLicenseBiology:
** It is not explained how a person must be "prepared" for the environment of the time period before being sent there. And once prepared, can never return. Mr. Atoz insists that one who is not prepared will die soon after arriving in the time period. This is the only instance of time travel where this was an issue.

* YouCantGoHomeAgain: Zarabeth tries to convince Spock and Bones this. The judge tries to convince Kirk this. Operative word: "tries".

* 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.

* 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 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.]]

* 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.

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.

Notably also the last episode of the series if you put them in chronological order by Stardate. It's generally considered one of the highlights of the often-shaky third season, and a much better series finale than [[Recap/StarTrekS3E24TurnaboutIntruder "Turnabout Intruder"]], although at that time television series rarely had an episode written as a series finale, but simply a last episode to air.

* 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.