[TNG] Season 3, Episode 17: Sins of the Father
The writers decided it was time to make Worf really awesome. Previous to this Worf had been getting some nice moments here and there, slowly growing beyond the punchline he originally was, but Sins of the Father is the first big step in establishing Worf as a great character. Beyond the character work, this episode creates a bigger, more robust and fascinating Klingon society that hadn’t really been seen before. It’s all done through a mystery that has numerous suspenseful turns.
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Read more at Memory Alpha
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Notes:

A Klingon acts as first officer of Enterprise in the officer exchange program. He pushes Worf’s buttons by being really nice to him.

Turns out it’s his little brother. Their father is being accused of being a traitor. Worf must go to the Klingon home-world and challenge the ruling.

If the challenge is rejected, Worf has to face punishment for his father (death of course). He thinks it would be a good idea if his little brother hide his relation

It’s revealed that the real traitor was Duras, but his family is too powerful, and ruling against them would cause a civil war. They figured no one would challenge the ruling against Worf’s father.
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