While chasing a ship that's going waaaaaay too fast (ie, Warp 36), the Enterprise enters a universe where time runs backwards.
Kirk and company enter an anti-matter universe where time, words and life of stars runs backwards. They eventually start aging in reverse, with the only ones able to command and pilot the ship home being Spock, due to his Vulcan physiology, and the elderly Commodore April and his wife.
This episode provides examples of:
- And the Adventure Continues: This was the last voyage of the TOS crew's Five-Year Mission before the movies.
- Continuity Nod: Commodore April refers to Kirk's previous mission to Beta Niobe, which occurs in TOS: "All Our Yesterdays". Kirk later refers to Minara, a star referenced in TOS: "The Empath".
- The process of using the transporter to revert Kirk, Spock, Bones, Uhura, and others back to their original selves, using the transporter's last-remembered stats on their bodies, was last used in TAS's "The Lorelei Signal" and "The Terratin Incident" (to revert old age and size shrinkage, respectively). It would also get used again a couple of times, in "Unnatural Selection" and "Rascals".
- Grand Finale: Of Star Trek at the time. Then The Motion Picture happened...
- Happily Ever After: Even though we never see them again its implied that Commodore and his wife live out a happy retirement.
- Negative Space Wedgie: How they enter/exit the anti-matter universe. It happens when they tractor Fives ship and can't release it when she entered.
- One Size Fits All: The Enterprise's crew rejuvenate, and their uniforms shrink to fit.
- Remember the New Guy?: Commodore (formerly Captain and commander of the Enterprise) Robert April. However, note Genius Bonus in the YMMV tab.
- Sdrawkcab Name: In a bit of clever writing note, the name Arret is also "Terra" spelled backwards.