"Duet" is quintessential DS9. It's an issue-oriented episode that is brilliantly characterized, with some absolutely riveting performances.
The plot centers around a Cardassian named Marritza (Harris Yulin) who "happens" upon the station as a passenger on a passing ship. Kira promptly arrests him for being a war criminal and throws him in a cell. You see, he has a medical condition that he could only have acquired at a Bajoran labor camp named Gallitep. This labor camp was also the site of horrific Bajoran treatment at the hands of Cardassian atrocities.
As far as Kira and the Bajorans are concerned, any Cardassian at Gallitep is guilty. But a mystery arises concerning the Cardassian's identity—with a number of clues that don't add up—and Kira begins a search for the truth. The resulting dialog between Kira and the Cardassian pulls no punches in either content or delivery. Evidence indicates that Marritza is really Gul Darheel, the man who actually ran Gallitep and made it his mission to terrorize "Bajoran scum." Suddenly Kira finds herself face to face with one of the most hated Cardassians Bajor has ever known.
Nana Visitor delivers a powerhouse, emotional performance. Even better is Harris Yulin's turn as Darheel, whose absolute tour de force display of acting brings the raving, menacing, downright evil Cardassian frighteningly to life, with such lines as "What you call genocide, I call a day's work." Odo's subsequent investigation of the Cardassian's identity brings Dukat into the plot with a great deal of sensibility.
It turns out that Darheel is really Marritza posing as the Cardassian criminal (who has been dead for years), trying to martyr himself so the Cardassian government will be forced to acknowledge its guilt for the Occupation—a moving display of self-sacrifice for the sake of progress on all ends. "Duet" is all substance, completely engrossing in its conveyance, and it also features a tragic ending. It's one of the best moments in the entire series' run.