TNG Season 3 Episode 17: Sins Of The Father

Mogh, father of Worf is accused of betraying the Klingons to the Romulans, and thus instigating the Khitomer massacre. Starfleet’s only Klingon officer isn’t going to just let it lie, only to find he has a brother he knew nothing about.
This is the first time Worf interacts with the Klingon High Council, at least on-screen, and comes into conflict with the Duras family for the first time – the actions in this episode would reverberate through Next Generation, DS9, and into Star Trek: Generations.

Plot

As part of the Federation-Klingon officer exchange program, Klingon Commander Kurn (Tony Todd) has requested to be placed aboard the Enterprise as first officer. His typical Klingon command style aggravates the crew, particularly Lieutenant Worf (Michael Dorn). Worf confronts Kurn alone, where Kurn reveals he is Worf's younger brother; when the rest of Worf's family left to the Khitomer colony, he was left with Lorgh, a friend of their father Mogh. Kurn was raised as Lorgh's son, unaware until recently of his relation to Worf. Kurn tells Worf that Mogh is being charged posthumously as a traitor by Duras (Patrick Massett), the son of Mogh's rival, in the Khitomer massacre, which will mar the Mogh family name for seven generations. Worf requests an urgent leave of absence to defend his father's honor. Captain Picard (Patrick Stewart) believes that 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 homeworld so they may monitor the events. En route, Kurn volunteers to be Worf's Cha'DIch, a second to stand with Worf during the challenge. Worf agrees, but warns Kurn to not reveal his bloodline just yet.
At the High Council, Duras reveals evidence of Mogh sending Khitomer's defense codes to the Romulans. Worf challenges this, but is told privately by the aging K'mpec (Charles Cooper), the Klingon Chancellor, to drop the challenge and return to the Federation. Worf discusses this curious request with Picard, who also finds it strange and orders his crew to examine the evidence. Meanwhile, Duras has ambushed Kurn, aware of his Mogh bloodline, and attempts to get him to betray Worf. Kurn refuses and is seriously wounded in the ensuing fight, no longer able to support Worf in front of the Council. Picard accepts Worf's request to take Kurn's place.
The Enterprise crew finds evidence that the Khitomer logs have been modified and soon discover one more survivor of the massacre, Worf's nurse Kahlest (Thelma Lee). Picard is able to convince Kahlest, who knows Mogh was loyal to the Klingon Empire but does not know who the true traitor was, to help in Worf's challenge. Picard brings Kahlest to court and bluffs that she knows who the true traitor was, starting a heated dispute that is sure to end in needless bloodshed. Infuriated and as a means to halt the bickering, K'mpec calls Worf, Picard, Duras, and Kahlest into his private quarters and reveals the truth; the Council is well aware that Duras's father was the Khitomer traitor, but exposure of this, given Duras's high political position and capital, would certainly lead to an unwanted civil war within the already trouble-stricken Empire. The Council only accepted Duras's charge of treason against Mogh believing that Worf would not challenge it due to his Federation citizenship. To prevent further upheaval, K'mpec imparts that the Council will condemn Worf and Kurn, but Picard refuses to let this blatant injustice stand, thus creating a situation that could end the Klingon-Federation alliance. Worf, seeing what restoring his family's honor may cost, steps in and says he understands what he needs to do, that the only course of action for Worf is to accept a discommendation, tantamount to admitting his father's guilt; in exchange, the knowledge of the proceedings, including Kurn's true-bloodedness, will be undisclosed. Back in the council, all of the assembled Klingons, including a reluctant Kurn, ceremonially turn their back to Worf in disgrace, and he and Picard silently leave the hall.