10 Most Important Episodes Of Star Trek: The Original Series

Tomorrow Is Yesterday follows the crew as the Enterprise is accidentally sent back in time to 1968 Earth, where their presence immediately discovered. They then realize the potential damage they are causing to the time stream and how disastrous it could potentially be if they don't put things back to normal.

Along the way, they manage to inadvertently make the situation worse, before ultimately resolving it just in the nick of time (no pun intended) and returning to their own era.

First and foremost, this is the very first episode of Star Trek to feature time travel, which is already rather significant in its own right. However, in doing so, Tomorrow Is Yesterday also laid the groundwork for other Star Trek stories yet to come.

This episode established the "slingshot effect" (also known as the "light-speed breakaway factor") in the Star Trek universe, which is a method of time travel that involves whipping a spacecraft around a star in such a way that an artificial wormhole is created.

This method would not only make a resurgence in a later episode of the show, Assignment: Earth, but would also go on to play a major part in the plot of Star Trek IV: The Voyage Home, which is largely considered to be one of the franchise's best films.