S3E17 · Sins of the Father

As part of the Federation-Klingon officer exchange program, Klingon Commander Kurn asks to serve aboard the Enterprise as first officer. His command style aggravates the crew. Lieutenant Worf confronts Kurn, who reveals he is Worf's younger brother: when Worf traveled as a child to the planet Khitomer with his parents, Kurn was left with Lorgh, a friend of their father Mogh; after their parents were killed in a massacre by the Romulans, Kurn was raised as Lorgh's son. Mogh is being charged posthumously as a traitor by Duras, the son of Mogh's rival, for colluding with the Romulans in the Khitomer massacre.

Worf requests a leave of absence to defend his father's honor. Captain Picard believes Worf's actions as a Starfleet officer in his father's defense will be of significant interest to the Federation and directs the Enterprise to the Klingon capital. Kurn volunteers to stand with Worf as his second; Worf warns Kurn not to reveal his parentage, lest he be punished for Mogh's alleged dishonor.

At the High Council, Duras reveals evidence of Mogh sending Khitomer's defense codes to the Romulans. Picard orders his crew to examine the evidence. K'mpec, the Klingon Chancellor, privately urges Worf to drop the challenge. Duras ambushes Kurn, aware of his bloodline, and attempts to get him to betray Worf. Kurn, wounded in the ensuing fight, is now unable to support Worf; Picard takes his place.

The Enterprise crew discovers that the Khitomer logs were modified and find another survivor of the massacre, Worf's nurse Kahlest. Picard convinces Kahlest, who knows Mogh was loyal to the Klingon Empire, to help. Picard brings Kahlest to testify, bluffing that she knows who the true traitor was. K'mpec calls Worf, Picard, Duras and Kahlest into his office and reveals the truth: Duras's father was the traitor, but due to the House of Duras' influence over some of the council, exposing this would lead to civil war. The Council accepted Duras's charge of treason believing Worf would not challenge it.

K'mpec imparts that the Council will condemn Worf and Kurn, but Picard claims this injustice could end the Klingon-Federation alliance. Worf accepts a discommendation, tantamount to admitting his father's guilt; the knowledge of the proceedings, including Kurn's relationship to Mogh, will be undisclosed. In the council, the assembled Klingons, including Kurn, turn their backs on the disgraced Worf as he and Picard leave the hall.