"All Our Yesterdays" takes Trek outside its usual limits, the only episode to show no interior scenes aboard the Enterprise. No one from the gang of four appears either, though Scott gets a few desperate words in over the communicator. Without the ship or her crew to highlight, the story revolves around Kirk, Spock, and McCoy, alone in unusual circumstances as they are involuntarily separated to different points in the past of planet Sarpeidon. (The inhabitants there never achieved space travel, Spock says, yet temporal travel to a planet's past implies being transported to where that solar system, moving in space, should have been previously at an altogether different location, aka space travel.)

The librarian A-to-Z uses a machine called the Atavachron to prepare people for their time trips to safety, changing them at some unspecified cellular and brainscan level, whatever that means. There certainly are changes to all three of the landing party though only Kirk shows sign of weakening (only one guard to subdue him?). By and large the changes are most pronounced in the love-smitten Spock, with suitable excuse in the beautiful Zarabeth (memorably played by a stunningly near-naked Mariette Hartley), and enhanced by a subtle but moving score.

In fact the episode features an excellent supporting cast including the wonderful Ian Wolfe as the librarian Atoz, Anna Karen as the lusty Mort ("I'm game luv … where's lie-brar-ee?") and Kermit Murdock as the "helpful" Prosecutor. What further redeems this effort is McCoy's steadfast (if not life-threatening!) determination to get back to "his life back there" over the passionate objections of his devolving Vulcan friend. We get an alarming peek at a seldom seen side of Spock; note that as they get ready to jump back through the portal, Spock gives McCoy a shove towards it, apparently set to remain in the Ice Age with Zarabeth. (Nice touches with his makeup too. Arriving at Zarabeth's cave Spock suffers from exposure to extreme cold, and his ears and face has a flushed, green appearance.)

Watch for an interesting blooper early on as Atoz gets up from his bluish marble seat to join the landing party. When Kirk returns to that seat to look at the viewer, it now has a black cushion. The effects, including the time portal's shimmer, are simple but impressive, as is the exploding flare of the Beta Niobe nova as it consumes its satellite behind escaping Enterprise. Though the idea is clever, the technical details don't bear close inspection. No matter. It's memorable anyway.