As the end of the rambling final season draws near, along comes "All Our Yesterdays" to rebuild some of the series' dignity. In a genuinely inspired story concept, the people of a planet whose sun is going supernova are escaping death by transporting themselves into the past.
The site of the time jump is an intriguing "library" run by the pervasively indispensable Mr. Atoz (Ian Wolfe), who maintains an urgency that's as believable as it is humorous. Kirk ends up accidentally jumping into the past, where his attempts to return to the present land him in jail, accused as a witch. Upon trying to follow and locate Kirk, Spock and McCoy find themselves sent into the planet's ice age. Spock begins to undergo an emotional change, as being sent so far into the past has caused the ancient undisciplined side of Vulcan to emerge. Spock's situation allows him the rare opportunity to fall in love with the banished Zarabeth (Mariette Hartley), but also reveals the hidden darkness of Vulcans that is buried beneath the logic, intellect, and control.
This episode is enjoyable as a character study and as an efficiently flowing story. It's entertaining, nicely crafted, and leaves one pondering the "what ifs" when it ends.