The much maligned third season of TOS had some poorly executed episodes, partially because of biting budget cuts and partially because of Fred Freiburger taking over from Gene Roddebberry as the producer. However this penultimate episode is both a cleverly written story and well made.
By keeping focus on the three central protaganists of Kirk, Spock and McCoy, the director is able to paper over the cracks of the budget cuts that plagued series three by cleverly utilising previously used sets and props with one of TOS's most popular gimmicks, the 'time travel' story. Admittedly this is a more narrowed use of that gimmick than before but by centralising the story on the three most beloved characters in TOS, director Marvin J Chomsky keeps the dynamic going just enough so you don't notice the plot holes or the budgetary compromises forced on him,such as no scenes set on board the Enterprise and the limited number of sets used.
Some series three episodes creak and groan, either through bad writing ('Spocks Brain'), or being overly ambitious with the resources available ('The Savage Curtain', 'The Empath'). 'All our Yesterdays' is a good example of a director who realises the confines he has to work with and makes them work to his advantage. Good use is made of the limited set ups he was allowed, the special effects budget was sparingly but well used and by keeping the focus on the characters rather than the scale, he hides the shortcomings well. This is possibly the best episode of season three for all those reasons.