One notably important aspect of TNG's third season was the way it expanded the canvas of the Trek mythos. TOS and the first two seasons of TNG felt more like a western in space (uncharted frontiers, etc.), but with this season, the Trek universe began reinventing itself as a place containing sprawling civilizations and a political landscape that was tangible in a way that we had not perceived before.
"The Enemy" and "The Defector" demonstrated that via the Romulans, and now "Sins of the Father" demonstrates it by providing a look at Klingons not simply in isolated obscurity, but as a society with cultures and (corrupt) politics. Kurn (Tony Todd) boards the Enterprise in the exchange program as payback for Riker's visit to the Klingon ship in "A Matter of Honor." At first the episode looks like "A Matter of Honor" in reverse, but Kurn soon reveals that he is Worf's brother, separated from the family before their parents were killed by the Romulan attack on Khitomer over 20 years ago. The Klingon High Council is planning to scapegoat the Khitomer massacre on their father, Mogh, alleging he betrayed the Klingons by supplying the Romulans with intelligence. Only by standing before the council and proving his father innocent can Worf restore his family's honor (at risk of death to himself, if his father is deemed guilty).
The episode does not play out predictably, which is one of its pleasures. It defies brief synopsis. Suffice it to say that through a series of twists, turns, attempted killings, and political cover-ups, Worf finds that he must accept discommendation for the crimes falsely pinned on his father. It's the only solution that will protect both him and his brother from execution while keeping the High Council from collapsing into a civil war over the uncovering of the true traitor — the father of Duras (Patrick Massett), whose family has too much power to be openly accused. Picard's personal involvement in this affair works because it allows us to enter these proceedings through a relative outsider's perspective and gain a better understanding.
"Sins of the Father" offers a lot to sink your teeth into and reveals more complexity to the Trek universe. It begins a storytelling tradition of Worf's responses to Klingon political corruption that would rear its head frequently all the way up to DS9's "Tacking into the Wind." (The method of this episode also would influence the intrigue-based storytelling that fueled many early Bajoran-themed stories on DS9.) It reveals Worf as a Klingon whose selfless pledge to protect the Klingon Empire is admirable, particularly seeing as the Empire sees little reason to return the favor.