“Duet” is episode nineteen of season one of Star Trek: Deep Space Nine.
A ship docks at the station and one of its passengers suffers from a rare disease called Kalla-Nohra. Kira is interested by this, as the only sufferers are those who were at a Cardassian labour camp during a mining accident, a camp that she helped liberate. However, the sufferer isn’t Bajoran – he’s Cardassian.
His initial claims that his disease is a different one are quickly proved to be false. Which means that Kira believes that he is a war criminal responsible for some of what happened at the camp. More claims are also proved false, and then the Cardassian states that he was the Gul in charge of the camp, and that he was proud of what he did.
However, things seem a little odd. The fact that he passed through DS9 where he would be discovered makes no sense, and Odo also discovers that there may be elements of his story that aren’t true. So, is he a war criminal or not? And if he isn’t, why would he pretend to be, and get himself captured so easily?