"Tomorrow is Yesterday" is the second episode that deals with a time-travel premise. In this episode, a space anomaly sends the Enterprise to the 20th Century Earth, and Kirk and his crew try to go back to the future without altering the past. Of course, all sort of complications arise. Not as good as the later "City on the Edge of Forever", but well made and interesting. It is actually very similar to the feature length movie "The Voyage Home (1986)". Like most shows that deal with time-travel, the story does not make much sense, but this show is an excellent example of how far the series had come in a relatively short time. Very enjoyable.