For some reason Voyager had a knack for getting a lot of its best concepts out of the way early on, and "Tuvix" is another example of that. Simply put, this is the episode where Neelix and Tuvok are merged into one body thanks to a transporter accident.
Like some of those old original series adventures where Kirk was seemingly subject to just about every conceivable transporter accident (except for creating a permanent clone of himself, which instead happened to his Next Generation surrogate Will Riker), one of Star Trek's signature elements takes center stage, with its own brand of ethical dilemma, something the franchise is known for but rarely actually gets credit for, and all anyone cares about is whether Tuvix should be allowed to continue his own life rather than split back to his component personalities.
The real trouble is that combined, Neelix and Tuvok seem to be better than they ever were apart. Hell, they've even made peace with themselves! It's Janeway who makes the decision, ultimately, to undo the accident, and it's another in a series of controversial moves on her part that's liable to make those who already hate her think that she and Voyager as a whole is one vast mistake that only serves to bastardize the franchise.
But as I've said, ethical dilemmas are old hat for Star Trek. Most of them involved weird alien cultures, originally, and rarely involved personal consequences for main characters, but that's the way it was done in that TV era. If Voyager hadn't done an episode like this, not only would it be impossible to respect it at the time, but there'd be no defending it now. But "Tuvix" exists, and it doesn't pull any of its punches, either on a first or fourteenth viewing. It defines the show's legacy as much as any episode, and it's an early example that the series wasn't afraid either of its own potential or of what anyone would think about it in relation to the decades of interest fans had already invested in other characters. So many fans still try to claim that Voyager played it safe. "Tuvix" makes a joke out of that, and the contention that something like BattleStar Galactica, just because it was grim and gritty, war more daring. It's more daring to challenge characters in multiple ways, rather than the same one, repeatedly, folks. "Tuvix" does that in a single episode. That's the strength of the Star Trek formula, which some fans would have you believe as being a dirty word.
franchise * series * essential * character
Memory Alpha summary.