Time-travel plots seem to fall into two broad categories. The first is a variation on Greek tragedy, where the very attempt to change past events actually itself causes what it was meant to prevent. (I like to tell my students that Oedipus Rex is actually the first time-travel plot.) The second is not so fatalistic — it turns out that you really can change the past, but normally the way the plot works out is that everyone decides that the best thing is to try to change things back the way they were. (The movie Primer is a bit of an outlier, but I think that it basically falls into the second category, insofar as it’s a cautionary tale about unintended consequences and hence implies that simply leaving well enough alone would have been preferable.)
In the latter kind of plot, the writers have to face a metaphysical decision, namely, whether the time-travellers retain a kind of “pointer” back to their original timeline. If they do, you can see phenomenon like the character starting to “fade out” as the odds of their ultimate existence decrease (perhaps most famously in Back to the Future). If they don’t, then they simply exist in their new time-position alongside everything else, and so there is less room for paradoxes like changing the timeline so that you never actually went back in time in the first place, etc.
This brings us to Star Trek. Generally speaking, it seems that the approach to time travel across the franchise has been in the second category on both counts — you really can change the timeline (though you shouldn’t), and you simply exist in your new time-position without any kind of “pointer” to where you came from. They also take a kind of Hegelian approach to the issue, focusing solely on world-historical events. There is no “butterfly effect” problem to deal with — as long as the broad outlines of past history go the right way, history turns out like it should. You can see this on several Deep Space Nine episodes, such as the one where Sisko stands in for the peaceful protestor who changed everyone’s perceptions of the internment camps for the unemployed and the one where they go back to the Tribble episode of the original series.
And yet sometimes they break these implicit rules, as in the episode where the DS9 crew encounter a human colony made up of their own descendants and face the choice of whether to duplicate the accident that sent them 200 years into the past — or else their descendants wouldn’t exist! As far as I can tell, this isn’t how it should have worked. The versions of themselves who travelled back in time simply existed in that timeline, and they don’t “need” their later counterparts to travel back again.
Of course, I may be attributing too much coherence to their approach to time travel given how massively nonsensical the Next Generation series finale was.
What about you, readers? What do you think about time-travel plots?