Klingon Commander Kurn is temporarily assigned as first officer aboard the Enterprise as part of an exchange program. Kurn's command style alienates several crewmembers and ultimately leads Lieutenant Worf to confront Kurn regarding the latter's behavior. After Kurn provokes Worf almost to the point of violence, he reveals to Worf that he is Worf's brother. When Kurn and Worf were children, he was left with Lorgh, a friend of their father's, when the family went to the Khitomer colony. Believing their family wiped out in a Romulan attack, Kurn was raised as Lorgh's son.
Kurn tells Worf the Klingon High Council has charged their father Mogh with treason. Honor demands that one of the sons of Mogh appear before the council to challenge the accusation. Kurn tells Worf that as the eldest brother the challenge is Worf's to make.
Duras, the son of Mogh's rival, has accused Mogh of aiding and abetting the attack on Khitomer. Worf requests a leave of absence in order to travel to the Klingon homeworld and defend his family name. Arguing that Worf's actions will reflect on the Enterprise and the Federation, Picard orders the Enterprise to the Klingon homeworld. Kurn volunteers to be Worf's Cha'DIch, in human terms a second. Worf orders Kurn not to reveal his bloodline.
In the High Council's Great Hall, Worf makes his challenge. Duras states that the capture of a Romulan ship led to the discovery of records indicating Worf's father sent Khitomer's defense codes to the Romulans.
K'mpec, the Klingon Chancellor, advises Worf to drop his challenge, saying he should let the prosecution go forward and return to his life in the Federation. Worf's challenge will "be forgotten". When Worf questions him about this strange request he refuses to elaborate.
Duras ambushes Kurn, telling him he is aware of Kurn's bloodline. When Kurn refuses to betray Worf he is attacked and seriously wounded. Worf tells Picard of K'mpec's offer, shocked that a Klingon would ask him to put aside his honor. Needing a new Cha'DIch, Worf asks Picard to stand with him.
Data, La Forge and Riker analyze Federation and Klingon sensor logs from the Khitomer attack, and ultimately determine that "someone has been rewriting history". Reviewing medical files from the Intrepid, which rendered assistance after the attack, Dr. Crusher finds another survivor of the massacre: Worf's nurse, Kahlest.
Picard locates Kahlest, but she refuses to aid Worf. She verifies that Mogh was loyal to the Klingon Empire; he went to Khitomer hoping to uncover who the actual traitor was. After getting ambushed by Duras' men, Picard convinces Kahlest to return with him.
Returning to the Great Hall, Picard tells the Council he has new evidence. In K'mpec's private chambers, it is revealed that Duras' father was the traitor. K'mpec admits that the Council would rather not reveal this information, and dismisses Kahlest. He tells Picard and Worf that the Council chose to conceal the truth because, with Duras' political power, a civil war would ensue if the truth were known. The council did not know Worf would challenge the ruling. Picard's evidence will not be presented, and the judgment must stand: Worf and Kurn will be condemned. When Picard refuses to allow the execution of a member of his crew or turn Kurn over to them. K'mpec replies that Picard's defiance could end the Klingon-Federation alliance.
In exchange for Kurn being spared, Worf accepts discommendation in open council, tantamount to admitting his father's guilt.