Suffering a personal spiritual crisis that causes Worf to be late for work one day (an offense which Picard finds to be transgressive to a bizarre degree — maybe only slightly less severe than killing Duras in "Reunion"), Worf requests a leave of absence to go be with other Klingons and immerse himself in his spiritual side, something he has found gnawing at him ever since his experience with the young Klingons in "Birthright, Part II." He goes to a colony on Boreth, where Klingons are awaiting the return of the ancient warrior Kahless, who has been prophesized to return. While sitting in a trance after several days or weeks or however long it takes for a Klingon's head to clear, Worf witnesses Kahless materialize before him, in the flesh. Later, Worf engages the outspoken Kahless (Kevin Conway) in a bat'leth fight. Because he's annoyed. Or maybe because it's fun.
And we've just barely gotten started. "Rightful Heir" is nothing if not ambitious in its storytelling, even if it threatens to ascend into the stratosphere of the absurd. The story at first seems like it's going to be another slog through ponderous Klingon mumbo-jumbo much the way the juiceless "Birthright, Part II" was, but Ron Moore shows here why he earned his reputation as the Klingon Guy; "Rightful Heir" has plenty of Klingon Claptrap, yes, but it also gains steam after the first couple acts with the much-needed juice, political shenanigans, and earnest dialogue. What starts as an out-of-left-field crisis of Worf's spirituality becomes, by the end, a battle for the hearts and minds of the Klingon Empire (albeit one told via microcosm on a few sets on the Enterprise).
The literal return of Kahless manages to pose the question of resurrection and prophecy in sci-fi allegory terms: If, say, Christ claimed to return from the dead after millenniums and was not accompanied by a hell of a convincing show of sound and fury, who would actually believe it to be true? Okay, maybe don't answer that, but I tend to think (hope?) most sane people — even believers — would be extremely skeptical. (I personally find the notion of belief in literal resurrection, religious or otherwise, to be silly on the level of believing in magic, but, hey, that's just me. I guess that would put me on Team Gowron for this story's sake — if not for all the political corruption, of course.)
The crucial element of success here is the story's suggestion of sprawling consequences for the Klingon Empire, as Kahless' return implies the dawn of a new era of leadership — but one that Gowron is not simply going to step aside and cede. Gowron engages Kahless in a bat'leth battle, and Kahless loses, which goes against the prophecy of Kahless' greatness. Worf (and, by storytelling microcosm, many others) begins to lose his faith, suspecting that political manipulators Koroth (Alan Oppenheimer) and Torin (Norman Snow) may be manipulating the entire situation for their own political power play — which it turns out they are, because they actually created Kahless as a clone from the long-dead real man's preserved DNA. (While the Klingons are allowed to believe in the supernatural, the supernatural itself does not actually exist here, this being Star Trek.) This actually proves to be an interesting story twist; it's a prophecy come to life because of science. But how much resonance does Kahless hold for the empire? Enough to divide it, it would seem.
Worf's proposed solution to this complicated quagmire is one of compromises that considers the importance of symbols like Kahless alongside the pragmatism of the political realities. (And I liked the way Worf's conversation with Data, of all unlikely people, ended up helping Worf come to his decision.) If you like Klingon politics, you will probably like "Rightful Heir," which is ultimately as intriguing as it is borderline ridiculous. And it's got the juice.
Footnote: Ron Moore also seemed to be trying out lines for later use here: Worf's "And if you do not tell me what you have done, then I will kill you right here!" sounds a lot like his line to Picard in "First Contact," which I still love for its gloriously theatric delivery: "If you were any other man, I would KILL you where you stand!"