Sometimes when a series last for a long time, it becomes easier to forget some of the notable episodes from the early seasons. It's worse for a Star Trek, because it's assumed that because Next Generation and Deep Space Nine took a while to discover their sea legs, it was common for all of the series. It's worse than you imagine because Voyager pretty much stayed the same throughout its seven seasons, which by some reasoning means it never improved and therefore was never worth watching. (Enterprise changed drastically in its third and fourth seasons, but was always worth the ride. The original series, meanwhile, achieved iconic status in spite of its third season.)
All of this is to say that "Non Sequitur" deserves to be remembered as one of the finest episodes in the franchise, not just the series. One of the sad ironies here is that Harry Kim's best moment outside of "Timeless" has essentially been lost all these years, meaning that just like Chakotay, his legacy has been obscured by an assumption that really needs to be put to bed at this point. This is a prime example of how creatively potent the second season of the series really was.
The basic outline is that Harry has been pushed into an alternate timeline, and so this is pretty much a reset episode, so that at the end, none of it really matters. Except most of the franchise reset episodes that fans casually deride (except for their personal favorites) are actually some of the best episodes of the franchise (one of the movies, even is a reset episode, The Voyage Home, and until 2009 that was the one that grossed the most and was the most popular, at least in pop culture terms, because most fans still stubbornly cling to Wrath of Khan).
What do we learn here? We learn what life would have been like for Harry had he not gotten the Voyager assignment. He would have been a desk jockey, just another brilliant engineer Starfleet usually sticks on starships to work on the problem of getting warp engines to behave. Except, just as Reginal Barclay would later prove, these desk jockeys can sometimes come up with unexpected innovations. (Starfleet is remarkably complacent most of the time, as best evidenced by its foolish decision to give up cloaking technology because of an agreement with Romulans, and so when it finally does have some of it at hand, it can't figure the durn stuff out on its own, whether onboard the Pegasus or Defiant.)
Harry's primary interests, however, are figuring out how he got here and hanging out with his fiancee, who's a certifiable hottie. Why does he want to leave this behind again? Because he realizes that things are actually better for him, especially since Tom Paris is also here, in his proper timeline. Paris never got his shot at redemption, and is now a bum who's slumming it just as much with freedom as he did on a penal colony. He's directionless, and Harry discovers that he is, too, without the context, the meaning of helping a lost crew get home, no matter how small his contributions.
Anyway, half the treat is the rare visit to Starfleet HQ in San Francisco, which at some point should have popped into a franchise executive producer's head as a brilliant place to set a whole series. This is probably one of the best opportunities to see what that might actually be like (alongside "Homefront"/"Paradise Lost" and a few of those later Barclay appearances I was alluding to earlier, not to mention "The First Duty," where we see Robert Duncan McNeill in a Starfleet uniform for the first time).
"Non Sequitur" is appropriately named for so many reasons, but more importantly, needs to be rediscovered for the just as many reasons why it's relevant as a reason to love Voyager, despite everything you've heard.
franchise * series * essential * character
Memory Alpha summary.