Kurn says he was left behind when Worf and his parents went to Khitomer, and was unofficially adopted by their father's friend and not told of his true heritage until recently. Now, their father has been accused of betraying the Khitomer outpost, and as the oldest son, Worf must challenge the accusation if any are to do so. When Worf tells Picard this, and says that if his challenge fails, he too will die as a traitor, Picard refuses to grant leave, saying instead the ship will accompany them to the homeworld of the Klingon Empire. Later, Worf invites Kurn to be his cha'DIch (or second), but tells him not to reveal his heritage.
When Worf beams down to challenge, the son of his father's rival, Duras, claims he has forsaken his heritage. He then reads the charges, saying that Mogh sent the defense codes of the outpost to the Romulans seconds before the shields fell, and then tears off Worf's sash. Then, while Data begins analyzing all available data on the Khitomer massacre, the chair of the High Council, K'mpec, privately urges Worf to leave, promising that the challenge will be forgotten if he does. Some time later, Duras waylays Kurn with assassins, tells him he knows of his parentage, and when Kurn refuses to turn aside from Worf, stabs him.
Though Kurn will survive, he cannot be cha'DIch in his condition, and Worf asks Picard to serve in his stead, who accepts. Meanwhile, Data and company have found Duras's evidence: a transmission from Khitomer just before the shields fell, with Mogh's security code. However, information from a Federation ship that was nearby at the time shows that something's being hidden. As the council reconvenes, Bev announces she's located the only other survivor of Khitomer: Worf's nurse, Kahlest. Picard finds her in the city, and overcomes her initial refusal to testify, convincing her to come and help Worf.
When she arrives, K'mpec calls an immediate recess. Privately (with Picard, Worf, Duras, Kahlest, and K'mpec present), it is revealed that it was Duras's father who betrayed the outpost, but since Duras's family is very powerful (and no one ever expected Worf to make the challenge), Mogh was blamed instead. When they still say they cannot let this evidence be heard, and that Worf will still be condemned, Worf offers them something more effective than his death: he will accept "discommodation" (or being cast out) if Kurn's life is spared. They agree, and as Worf is cast out, Picard tells Kurn this was done so that one day he (or his children) can restore Mogh's honor.