This is the episode the original series never got around to, could probably have never done. It's the sequel to "Journey to Babel," which is to say Spock father Sarek's debut. "Sarek" is one of his final appearances, fittingly.
Sarek never came to peace with his son. In a way, as a Vulcan, as the traditional presentation of Vulcans go, he never could. (Incidentally, that's one of the reasons I love the 2009 Star Trek so much, because of that particular revision.) The only way he could was through Jean-Luc Picard (who in some of the final scenes in the episode go, has one of his most emotionally draining experiences ever).
The story of "Sarek" is one of those classic "final mission" stories. Sarek can no longer hide the ravages of a disease that is soon enough going to claim his life. He's losing control of his emotions, which for Vulcans is the ultimate indignity. He wants one last diplomatic success under his belt, though. He's become a legend of the Federation in his own right. He wants to preserve that legacy.
It's arguably the finest crossover episode in the franchise, where a character from one series appears in another. Part of the reason why is because it's so unexpected. Sarek, of all characters from the original series? He made one appearance in the series. He became better known for his appearances in the later films, Star Trek III: The Search for Spock, actually, the first time we get a hint of what he really feels about his son.
Sarek ends up being as strong a character as ever featured in the franchise, his few appearances notwithstanding, and this is his finest hour, which only figures. Totally divorced from the presence of his son, Sarek is forced to fend for himself. Failing or not, his dignity is strong. He makes himself a legend. If this had been his only appearance, Sarek would still have left the same legacy in the franchise. That's how good this episode is.
Yes, there's a clearer bridge in the later two-part "Unification," in which Spock himself appears, where Picard is able to share hidden truths between father and son, but it's better and more resonant here. Rare is the opportunity for someone to upstage Leonard Nimoy. In his absence, Mark Lenard does exactly that. Is that the only way? It doesn't matter. Lenard doesn't let you even consider that. One of the finest hours of The Next Generation features a character from another era.
If you want to understand Star Trek at all, this is a fine place to start.
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Memory Alpha summary