Two-part episodes are always a big deal in Star Trek, especially if they aren't a season finale/premiere, since that's what's most typical. Voyager started to make an art-form of it in the third season, but the best ones are in the fourth.
"Year of Hell" is the story that most fans expected the whole series to look like, and so if you're wondering what that would have actually been like, look no further.
Based on aliens first referenced in the third season episode "Before and After," this is the epic story of everything that could possibly go wrong, the ship and the crew completely degenerating, and not because of any enemy that has a particular problem with Voyager itself, but rather a regional problem that the series premise always had as a possibility, moreso than any other Star Trek crew might have expected, except maybe Enterprise.
It's Voyager as an opera. It's the later "Course: Oblivion" played out, and is uniquely effective, even if there's a reset bottom by necessity waiting at the end, BattleStar Galactica by way of Star Trek, why this was never done before and why even the third season of Enterprise waited until the end of the Xindi arc to finally duplicate it.
The weight of Janeway's command is a key element of the story, although this time it's nothing of the decisions she has to make (only the ones she has to make), as close as this series will ever get to a movie, and in fact the first time Voyager makes its own surrogate movies, when it became possible for Star Trek to present this kind of experience in its TV incarnations, which led to the epic Dominion War in Deep Space Nine and plenty more in Enterprise.
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Memory Alpha summary.