All Our Yesterdays ()
Kirk, Spock, and McCoy become trapped in the past of another world.
The Federation starship Enterprise arrives at the planet Sarpeidon, whose star is soon to go nova. Surprised to find the surface devoid of humanoid life, Kirk, Dr. McCoy, and Spock beam down to investigate. They encounter one last remaining resident, a librarian named Mr. Atoz. Aware of the imminent destruction, Atoz tells the landing party that he will soon escape to rejoin his family. Atoz shows them the Atavachron, a machine with a time portal. They hear a woman scream. Kirk instinctively runs through the portal, and McCoy and Spock follow. Atoz tries to warn them that they were not “prepared.”
Kirk finds himself in a period similar to 17th century England. At the same time, McCoy and Spock travel back 5,000 years to Sarpeidon’s ice age. They cannot locate the portal, but the three can speak to each other. Spock surmises that the Sarpeidons have all escaped to their planet’s past.
The woman who screamed is a thief, and the policemen who come to arrest her hear Kirk speaking to his friends and suspect him of being a witch. In the jail, Kirk mentions the Atavachron to the interrogating prosecutor. This startles him, and Kirk suspects he is also from the future. Kirk escapes his cell by luring his jailer close and overpowering him. The prosecutor confirms his suspicions but explains that returning to the future would be fatal as time travel preparation involves changes to the traveler’s biology.
Kirk tells him that he went through no such preparation. The prosecutor informs him that an “unprepared” person cannot survive in the past for more than a few hours, just as a “prepared” person cannot survive in the present. The prosecutor leads Kirk to the portal, and Kirk returns to the present.
McCoy and Spock are saved by Zarabeth, a woman who takes them to a sheltering cave. Spock begins displaying uncharacteristic emotion and continues carrying McCoy when logic would dictate he abandon him. He falls in love with Zarabeth, and he reacts with anger at McCoy’s racial slurs. As McCoy recovers from severe exposure, Zarabeth explains to Spock that she too is from Sarpeidon’s future, but a tyrant banished her to this era, in which she is the only extant humanoid. She also explains that the Atavachron is a one-way trip. Spock accepts this and becomes increasingly hostile to McCoy’s continued desire to return to the present.
McCoy accuses Zarabeth of lying about their inability to return. She is desperate not to be abandoned to unending loneliness again. She says that Spock backs her out of emotional attachment. McCoy realizes that being brought 5,000 years into the past is causing Spock to revert to the barbarism of the ancient Vulcans. Realizing the emotionalism of his behavior, Spock asks Zarabeth again about the portal. She admits she doesn’t know the effect of a return trip on him and McCoy.
Back in the library, despite Kirk’s explaining that they are not from Sarpeidon, Atoz stuns him and tries to force him back into the portal. Kirk awakens, overpowers Atoz, and forces him to help find McCoy and Spock. Eventually, Kirk can talk with the two. Unwilling to leave Zarabeth and still unaware that he will die soon if he remains in the past, Spock tries to send McCoy through the portal alone. However, because McCoy and Spock went through the portal together, neither can go back without the other. Upon their return, Atoz hurries through the portal to his chosen era. Spock reverts to his usual self, and with little time to spare, they are beamed back to the Enterprise.