Chances are the episode title "Tomorrow is Yesterday" doesn't sound too familiar, unless you're a true Star Trek geek. But the episode itself has a couple of franchise milestones attached to it.
The first is that it's the first time travel episode, and not only that, but the first time our characters from the future end up in our contemporary times. For a series trying to save a few bucks, it's a little surprising this didn't happen sooner.
The second is that the method of time travel is the "slingshot effect," the very maneuver later and more famously featured in Star Trek IV: The Voyage Home (the one with the whales).
So a lot of history being made. The episode itself becomes a little like some more familiar stories, a primitive civilization that is not at all prepared to deal with Kirk and crew's existence. (All these are a little like the reverse of the "god box" scenario; since I'm writing these reviews in groups, there's good reason why there are a lot of "god box" references in them lately.) Voyager's "Blink of an Eye" also serves as a nice callback, among other examples. That's the one where Janeway's ship becomes lodged in orbit of a planet whose rotation is faster than normal. The whole history of that world plays out in moments. "Tomorrow is Yesterday" is like that, but the B-movie version, the bare bones, more forgettable one, unless you're truly partisan to the series (as a lot of Star Trek fans remain).
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