Worf is uncharacteristically late for a shift because he was distracted by a ritual to invoke the fabled warrior Kahless.
He tells Picard that since meeting the young Klingons who grew up with no knowledge of their heritage (s6e16&17 “Birthright“), he has felt “empty,” and feels the need to test his faith.
Picard is simultaneously strict and empathetic, calling Worf’s actions “Inexcusable… and understandable.” He grants him a leave and tells him not to come back until he’s ready to perform his duties properly.
In a shrine on the holy planet Boreth, Worf joins a group of Klingons who chant, seek visions of Kahless, and await his return.
After 12 days in the shuttle and at least 10 days chanting, Worf has had no vision and tells the shrine leader Koroth that he’s ready to give up, but the stories about Kahless still have an effect on Worf, so he gives in to Koroth’s request to try one more time.
And wouldn’t you know it — a figure appears, saying that he is Kahless, and that he has returned to lead his people. He recognizes Worf, remembers what he told Worf in a past vision, and knows all the lore the Klingons expect him to know. Worf verifies with his tricorder that Kahless is Klingon, but remains skeptical. When Kahless hints that he may have to fight the established leader Gowron for the loyalty of the Klingon people, Worf challenges him to combat. Worf gains the upper hand, but Kahless is so good at rallying the crowd that Worf backs down.
On the Enterprise, Crusher runs through a list of the various sci-fi tropes that we should suspect by now (shape-shifter, replicant, surgery), but Picard notes their orders are simply to transport Kahless to the Klingon home world, not to test his story.
Worf suggests they might also consider that he is what he says he is. But Gowron comes aboard determined to unmask an impostor, bringing a ceremonial knife stained with the blood of the historical Kahless. Crusher’s sickbay gizmos confirm an identical match.
Worf is sold, but Gowron is convinced this is part of a plot by the shrine leader Koroth to seize power. After Gowron defeats Kahless in combat, Worf halts the killing blow.
In private, Koroth admits that the Kahless who returned is a clone, who had been given the memories of Kahless as reported through the sacred texts. Koroth says his inention is to bring hope and honor to the Klingon people, and in fact some of Gowron’s men have come to believe in Kahless; it does not matter to them that they saw Gowron defeat him in battle.
As Worf deals with his crisis of faith, comfort comes from an unlikely source — Data. The android says after he was told he is an andriod, he he has chosen to believe that he is a person, with the potential to grow beyond his programming, and it has given his life meaning and direction. “I made a leap of faith,” he says.
Inspired by Data’s unorthodox definition of faith, Worf brokers a clever compromise, which involves telling everyone the truth — that this is a clone of Kahless, created to teach and inspire the Klingon people. Worf plans to support naming the genetic heir to Kahless as the ceremonial emperor, but leaving the real power structure untouched.
All parties agree, and Worf has averted another civil war; but he’s still feeling empty.
The Kahless clone has already accepted his role as spiritual leader, bucking Worf up with a speech about the historical Kahless: “If his words hold wisdom and his philosophy is honorable, what does it really matter if he returns? What is important is that we follow his teachings.”