I'm curious how many episodes throughout Star Trek, from an in-universe perspective, either didn't happen or may as well not have happened, as in:

  • All the events are forgotten (like in Voyager's "Unforgettable")
  • Events are erased from the timeline (like that time loop episode in The Next Generation where Data sends the number 3 into the next loop)
  • Events actually happened to duplicates of the main characters (like the Voyager episode on a duplicate ship)

A little clarification: An episode "didn't happen" if, by the end of it, all or most of the events are set back to a point where they effectively didn't happen (or arc for that matter) usually due to the resolution. It doesn't count as "didn't happen" if the situation is resolved but the events just happened to occur in a different universe so that they don't matter.

This last bit is important because it excludes "Inner Light", "Parallels", and mirror universe stories for the most part. (I forget if there are time travel ones.)

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    This is a good question! I think there could be points where interpretation is necessary, such as DS9's "Far Beyond the Stars" or TNG's "Inner Light," where the respective captains have a dream? In-universe, the captains really did have their dreams, but the dreams themselves weren't real events. Inner light makes it even more complicated by suggesting the dream was a historical record.
    – Turambar
    Commented Sep 8, 2016 at 12:02
  • Comments are not for extended discussion; this conversation has been moved to chat.
    – Null
    Commented Sep 9, 2016 at 19:31
  • I thought this was going to be a question about episodes that were conceived but never produced. Commented Sep 20, 2016 at 16:36
  • This question needs clarification because it asks for episodes which "didn't happen" but the answers, including the accepted one seem to indicate the intention of the question is merely about episodes wherein a significant portion "didn't happen", not the entire episode. Commented Nov 26, 2016 at 5:13
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    It's probably worth noting that, in the vast majority of cases where the big, shiny red reset button is pushed at the end of an episode, some artifact of the alternate timeline always remains, even if it's just a log entry or someone's memory of the events. These artifacts would not exist had the timeline not actually happened. The only episodes that I know of where all artifacts are definitively erased are Year of Hell I and II from STV, though there might be a very few others. Commented Sep 10, 2017 at 21:46

2 Answers 2



  • S1.28 - The City on the Edge of Forever (possible: only the people who went down to the Guardian planet witnessed the fact that McCoy created an alternate timeline which was then set back on course by Kirk and Spock; a tricorder recording captured at least some of the details of the alternate timeline)


  • S1.2 - Yesteryear (alternate timeline / characters wiped)


  • S3.15 - Yesterday's Enterprise

  • S4.14 - Clues (possible option here - everyone forgets except Data, although the events still happened)

  • S5.18 - Cause and Effect (the Enterprise escapes the time loop and is not really destroyed)

  • S6.25 - Timescape (Time goes backwards after a warp core breach begins, so the breach never happened)

  • S7.11 - Parallels (although the alternate timelines may continue to exist in parallel with the main one, at the end of the episode Worf goes back in time and the events caused by his timeline-hopping seem to be cancelled out, at least from the POV of everyone else)

  • S7.25 - All Good Things (multiple timelines, duplicates wiped)


  • S4.03 - The Visitor (timeline is reset at the end of the episode)


  • S1.04 - Time and Again (the crew investigate a "polaric" detonation and wind up preventing themselves from creating it in the first place)

  • S2.05 - Non Sequitur (Harry Kim gets dumped in a timeline where he never joined Voyager but gets himself back restoring the original timeline. IIRC he still remembers it but for everyone else it never happened)

  • S3.21 - Before and After (Kes' consciousness travels back in time throughout her entire lifespan, Kes remembers all her "stops" but they are wiped out, and the next four seasons show that most of the "future" she sees plays out substantially differently)

  • S4.08, S4.09 - Year of Hell, Parts 1 and 2 (the events of both episodes are wiped from the timeline at the end)

  • S4.22 - Unforgettable (Alien species causes forgetfulness after they leave, only record is Chakotay writing on physical paper)

  • S5.6 - Timeless (Harry + Chakotay send a signal back in time to prevent Voyager's destruction. Only record is a transmission Harry sent back to himself)

  • S5.18 - Course: Oblivion (Copies of the crew from the episode "Demon")

  • S5.24 - Relativity (Seven of Nine is recruited by Capt. Braxton to travel to various points in time to prevent a temporal attack on Voyager. Possibly debatable as not all of the episode is wiped out but large chunks of it are - at least from the POV of Seven and the rest of the Voyager crew)

  • S7.11 - Shattered (Voyager is split into multiple timeframes from the ship's history with only Chakotay able to move between them. He succeeds in averting the original disaster and preventing the shattering taking place. Only Chakotay retains any memories of it)


  • S3E8 - Twilight (Archer is infected by subspace parasites which are destroyed in order to reset the timeline)
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    I find the process of how people get to answers as interesting as the answers themselves... but I think Inner Light qualifies as it having happened. Likewise Mirror verse events did happen, just not in our universe. Parallels I'd say is the most debatable, because those events all happened, but they're in a parallel universe where as Timeless just didn't happen, likewise that one where Voyager splits in two is another questionable one (especially Harry's situation). I think those are more a matter of perspective and opinion with regards to that and should be in a separate category.
    – Durakken
    Commented Sep 8, 2016 at 16:40
  • 1
    Can we add some kind of categorization separating events that definitely didn't occur (according to the criteria) from those that are more ambiguous? Commented Sep 9, 2016 at 1:46
  • 1
    So the answer is 14?
    – user14111
    Commented Sep 9, 2016 at 3:30
  • 1
    What about all the episodes between Demon (S4E24) and Course: Oblivion (S5E18)? If those all count, there's almost a complete season that "didn't happen".
    – Mast
    Commented Sep 9, 2016 at 10:53
  • 2
    Yesterday's Enterprise had meaningful changes in Prime. Alternate Tasha Yar went back with the "C" crew, got caught and had a half Romulan child (who looked just like her! cough). Course: Oblivion also actually happened in Prime. It's just there's no record of it because it was a tree falling in the forest with nobody around to see it...
    – Machavity
    Commented Sep 9, 2016 at 12:42

This probably isn't what the original poster had in mind, but there's another way to think about it:


  • Events are erased from the timeline....


It doesn't count as "didn't happen" if the situation is resolved but the events just happened to occur in a different universe so that they don't matter.

One way to answer this (and I'm not trying to be flippant) is "every episode and film in the franchise except those of Enterprise," if you consider the 2009 movie to be a giant reset button on the original timeline, making everything that happened after the Kelvin incident "un-happen." All episodes of Enterprise, off the top of my head, take place before the Kelvin incident. Though, by this logic, I suppose it means the mirror universe episodes of Enterprise did not happen, either.

On the other hand, if you consider the 2009 movie to be an alternate universe on a new, separate timeline, then this stance doesn't hold water, and this is probably what the original question presumes. It comes down to your opinion on time travel semantics/nomenclature/etc. and thus whether or not the existence of Ambassador Spock and the Narada by definition means they are on a second timeline in another universe, or a new timeline in the same universe. Generally speaking, Star Trek writers have not been entirely clear in regards to this, either on-screen or off-screen.

This is additionally paradoxically problematic with regards to episodes like VOY Future's End and DS9 Past Tense, where events seemed to begin in the future with characters traveling back in time to fulfill specifically predestined roles that would have affected both timelines. (First Contact wouldn't apply here, in my opinion.) But, again, this is all just another interesting angle to ponder.

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    "if you consider the 2009 movie to be a giant reset button making everything episode-wise that happened after the Kelvin incident "un-happen." - why would you consider this? Nobody associated with the making of that film series considers it a reset button on the original timeline. It is very clearly stated in and out of universe that it is a alternate and entirely seperate universe
    – NKCampbell
    Commented Sep 9, 2016 at 1:54
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    @NKCampbell: None of us can say whether this results in the eradication of the original timeline or not, or whether there is actually any metaphysical distinction between the two choices. Or whether it really was just an alternate timeline or something else. I see no reason to dismiss this possibility any more than to dismiss your run of the mill I-time-travelled-and-changed-the-past stories (such as "Timescape" or "Time and Again" or "Year of Hell" or, well, any of them really!). (cont.) Commented Sep 9, 2016 at 8:19
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    (cont.) Ultimately, whether an alternate timeline or even an alternate universe concurrently "exists" with ours is an entirely philosophical debate, and I mean that in the most literal sense. We just don't have a strong enough definition of the nature of "existing". This is "if a tree falls in a forest and nobody's around to hear it, does it still make a sound" territory. And, yes, I am aware that many statements both on this site and from producers categorically state "both timelines exist". Commented Sep 9, 2016 at 8:20

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