"Meld" is the definitive Tuvok episode, and it's a rival of "Amok Time" as a definitive Vulcan episode. In fact, it's Tuvok's "Amok Time.
"Amok Time," as any Star Trek fan ought to know, is the original series episode where Spock is suffering from the Vulcan blood fever, pon farr, and begins acting irrationally until he's allowed to go home and mate. Except it doesn't prove that easy. He's got a challenger to his intended bride, and has to fight for it. And it's not even that easy! He ends up having to fight Kirk, and actually "kill" him.
"Meld" is like that, but thoroughly Tuvok instead of thoroughly Spock. "Amok Time" happened because Spock served in Starfleet. "Meld" happens because Tuvok is lost in the Delta Quadrant. Tuvok was Voyager's tactical officer, but also served as the local sheriff, and as such when a murderer is discovered among the crew, it falls to him to discover the culprit. Like an episode of Law & Order: SVU, this does not actually solve the problem, because Lon Suder cannot control himself. So Tuvok offers to control Suder for him. He does this by melding with him and taking on his murderous impulses, like Picard agreeing to assume Sarek's emotions, and it doesn't quite work out as planned.
Tuvok has Vulcan control, but not enough. "Meld" is about the limits of self-restraint, of responsibility, and accountability. It's about Suder as much as it about Tuvok, but mostly, it's Tuvok's finest hour. It's another episode that challenges the perceptions of those who claimed it was unnecessary and fruitless to pursue Voyager as conceived, perhaps one of the best examples as to how the series lived up to its potential, in many and unexpected ways.
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Memory Alpha summary.