Original air date: March 14, 1969
The Power Trio 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 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. 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 The Three Musketeers. 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....
Notably also the last episode of the series if you put them in order by Stardate. At that time, television series rarely had an episode written as a series finale, but simply a last episode to air. This episode, with its funereal tone and focus on the relationship of the central trio, is generally considered one of the highlights of the often-shaky third season, and a much better series finale than "Turnabout Intruder", with an absolutely killer last line.
All Our Tropes:
- The Aloner: 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.
- Ancient Keeper: 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.
- Artistic License Biology:
- 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.
- Artistic License Physics:
- The idea that Sarpeidon would have been remotely habitable mere hours before its sun erupted is just about impossible — such a star tends to expand and contract, which would severely affect the climate of any orbiting planets.
- The Enterprise has to jump to "maximum warp" to Outrun the Fireball. In reality, a supernova (like everything else) expands at slower-than-lightspeed, so merely jumping to warp one would be all it took to outrun one.
- Bittersweet Ending: Poor Zarabeth. Poor Spock.
- Burn the Witch!: 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.
- Cool Gate: The Atavachron, a gate to the past.
- Cruel Mercy: The tyrant who scattered Zarabeth and her family didn't want to be guilty of killing them directly, so he exiled them by sending each to a different time period with the weapons, food, and other means they would need to survive. While it's implied he did this to all of them, for Zarabeth for certain he did this by sending her to a place and time where she would be entirely alone for the rest of her life.
- Did Not Get the Girl: Spock has to leave Zarabeth because it's the only way for Bones to return and she can't follow.
- Dissonant Serenity: Spock does not mourn having to leave Zarabeth permanently even after experiencing positive emotions all thanks to her for at least a few hours. Spock defends his underreaction that Zarabeth died tens of thousands of years before the time he is in currently, when taking into account the time travel portal it just happened seconds ago.
- Emergency Temporal Shift: The inhabitants of the planet Sarpeidon flee into the past before their sun goes nova.
- Exact Time to Failure: The Enterprise can calculate the supernova explosion to the second.
- Human Aliens: 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.
- I Will Only Slow You Down: 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.
- Literary Allusion Title: From Shakespeare's Macbeth: "And all our yesterdays have lighted fools the way to dusty death...." Not only a blatant reference to the episode's main conceit, but also foreshadowing of the Bittersweet Ending.
- Meaningful Name:
- What better name for a librarian than "A to Z"?
- The Atavachron, a portal which allows the planet's people to go back in time to live with, essentially, their ancestors (i.e., atavism). Lampshaded by Spock, though he doesn't get to define the "intriguing nomenclature".
- No Good Deed Goes Unpunished: Kirk hears a woman screaming and rushes to assist, and winds up trapped centuries in the past, accused of theft and witchcraft.
- No One Gets Left Behind: Spock, of all people, does this when McCoy collapses from exposure to the elements and tells the Vulcan to go on without him.
- No OSHA Compliance: The Atavachron's Portal Door 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".
- Nubile Savage: Spock and Bones meet one of these. It turns out she was exiled to the distant past via a time machine by a dictator.
- Offscreen Teleportation: In the teaser, the dramatic moment leading into the opening credits is Mr. Atoz apparently doing this, always turning up ahead of the landing party as it moves around the library. It's subsequently explained that he has duplicates of himself, created by some unexplained process, that help him run the library.
- Oh, Crap!: The judge has this reaction twice in the space of a few minutes—first when Kirk's mention of the library lets him know he's from the future, and second when, after attempting to get Kirk declared innocent, the redheaded thief and the Musketeers who brought her in testify to Kirk being able to call on "spirits"...meaning he's to be accused of witchcraft, something the judge can't dismiss without being thought guilty himself.
- O.O.C. Is Serious Business: Spock starts out getting visibly frustrated, telling Bones off for his constant jabs at his Vulcan heritage, and eventually escalating into a rage. He kisses a girl passionately. And he eats meat. It seems that all Vulcans have a loose psychic link with one anothernote , and since they are thousands of years in the past and all Vulcans are in a savage state....,
- Out-of-Character Moment: The original script Spock falling in love and kissing Zarabeth, but averted in the filmed episode at Leonard Nimoy's insistence. Instead, his behavior was hand waved with the explanation that, having gone back in time, he became like the savage Vulcans of that time.
- Outrun the Fireball: The Enterprise outruns a supernova at the end of the episode.
- Portal Door: the Atavachron, a machine that creates a portal door/wall to a time in that planet's past.
- Redhead In Green: The woman Kirk saves from a Musketeer. She even has hints of an Oirish accent.
- Samus Is a Girl: The Jawa with gigantism that saves Spock and Bones from freezing to death pulls back the hood to reveal a young Mariette Hartley!
- Sci-Fi Writers Have No Sense of Scale: 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.
- Stock Footage: The imagery our heroes watch include stock footage of a horse drawn carriage and a frozen wasteland.
- Sword Fight: Kirk gets in one with a Musketeer. He wins! He can sword fight with Musketeers, he can hold his own against the Gorn, but he can't take on an 80 year old librarian?
- Temporal Sickness: 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 get back in time.
- Twirl of Love: Spock's kiss with Zarabeth turns into this.
- You Can't Go Home Again: 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. 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.