Ironically, as William Shatner notes in his book "Star Trek Memories," this show was the source of some dispute since Leonard Nimoy noticed that no reason was given in Lisette's script for the reason why Spock was behaving in such an emotional way. Nimoy relayed his misgivings here directly to the show's executive producer, Fred Freiberger, that Vulcans weren't supposed to fall in love. (p.272) However, Freiberger reasoned, the ice age setting allowed Spock to experience emotions since this was a time when Vulcans still had not evolved into their completely logical present state. This was a great example of improvisation on Freiberger's part to save a script which was far above average for this particular episode. While Shatner notes that the decline in script quality for the third season hurt Spock artistically since his character was forced to bray like a donkey in "Plato's Stepchildren," play music with Hippies in "the Way to Eden" or sometimes display emotion, the script here was more believable. Spock's acting here was excellent as Freiberger candidly admitted to Shatner. (p.272) The only obvious plot hole is the fact that since both Spock and McCoy travelled thousands of years back in time, McCoy too should have reverted to a more primitive human state, not just Spock. But this is a forgivable error considering the poor quality of many other season 3 shows, the brilliant Spock/McCoy performance and the originality of this script. Who could have imagined that the present inhabitants of Sarpeidon would escape their doomed planet's fate by travelling into their past? This is certainly what we came to expect from the best of 'Classic Trek'--a genuinely inspired story.
Shatner, in 'Memories', named some of his best "unusual and high quality shows" of season 3 as The Enterprise Incident, Day of the Dove, Is there in Truth no Beauty, The Tholian Web, And the children Shall Lead and The Paradise Syndrome. (p.273) While my personal opinion is that 'And the children Shall Lead' is a very poor episode while 'Is there in Truth no Beauty' is problematic, "All Our Yesterdays" certainly belongs on the list of top season three Star Trek TOS films. I give a 9 out of 10 for 'All Our Yesterdays.'