A Kobheerian freighter transports a passenger onto the space station who has a medical condition known as Kalla-Nohra. Kira recalls that the only cases of this were the result of a mining accident at a Bajoran forced labor camp she helped liberate years ago. She goes to the Infirmary, and is taken aback when the patient is a middle-aged Cardassian male. He flees but runs right into Odo, who places him in a holding cell based on Kira's charge that, because he is a Cardassian with Kalla-Nohra, he had to be part of the military operation at the Gallitepp camp, and therefore a war criminal. When Sisko confronts the Cardassian, Marritza, about this, he claims that he does not have that particular condition, but one with similar symptoms. However, Bashir is certain that Marritza really does have Kalla-Nohra. Then, Sisko speaks with Kaval, the Bajoran Minister of State, who makes it clear that this is becoming a matter for his people, not the Federation, and if Marritza was at Gallitepp, Sisko will turn him over to Bajor — whether he wants to or not.

Kira pleads her case, and Sisko agrees to let her handle the investigation. Odo's background check confirms Marritza's claims about being an instructor for the past few years. Unconvinced, Kira interrogates Marritza, who eventually admits he was at Gallitepp — as a filing clerk. He claims there were no atrocities at the labor camp — just rumors started by the camp's leader, Gul Darhe'el, to create fear in the Bajorans. Growing tired of this, Marritza insists on being released, and hits a nerve with Kira when he tells her she doesn't care about truth — only vengeance.

The Cardassian Gul Dukat communicates with Sisko, but he refuses to release Marritza until his identity is confirmed. Later, in Ops, the officers examine the only image found in the Bajoran archives of Marritza at Gallitepp, but the Cardassian with that name in the picture bears absolutely no resemblance. Surprisingly, though, they do find a perfect match with another — Gul Darhe'el, the "Butcher of Gallitepp." Kira confronts Marritza, who admits to being the notorious leader and assuming Marritza's identity. With disbelief, she listens to him brag about the countless atrocities he committed, brushing aside Kira's threat of paying for his crimes, and proclaiming that he accomplished more than her resistance group ever did. Kira sees her words mean nothing to one who feels so superior.

Kira seeks solace by talking with Odo, who points out something very odd about Marritza's claims. How did he know to which resistance group Kira belonged? She confronts the Cardassian with this, and he claims that he was kept informed of everything regarding the resistance. Meanwhile, the suspicious Odo discovers Marritza was seeking information on Kira months ago; then a conversation with Gul Dukat reveals that not only is Gul Darhe'el dead, but he attended the funeral himself. Dukat allows Odo limited access to Cardassian files in order to get to the bottom of this. In the holding area, Kira and Marritza continue to debate, and he reminds her that he wasn't the only one who killed innocents — her terrorist attacks certainly murdered their share of Cardassian civilians. The discussion reaches a fever pitch when Odo pulls her away with his news. He isn't certain why yet, but he is sure Marritza wanted to be caught.

The evidence against Marritza mounts: Odo has seen Gul Darhe'el's death certificate; the labor camp leader wasn't on Bajor when the mining accident causing the Kalla-Nohra condition occurred; Marritza resigned from his teaching position two weeks ago and put all his affairs in order, he specifically requested passage to the space station; and Bashir has found enough medical clues to deduce Marritza apparently had his face changed to look like Gul Darhe'el. Kira tells him what she now knows, and finally, reduced to a tortured, saddened man, Marritza admits the masquerade. Kira moves to release him, but he begs to keep up the ruse, hoping that forcing attention to Cardassian crimes will force Cardassia to admit its guilt — and maybe bring about a new empire. Kira, not willing to let him sacrifice his life, prepares to return him home when a Bajoran man comes through the crowd in the Promenade and plunges a dagger into Marritza, killing him. The Bajoran, Kainon, claims that the fact Marritza is a Cardassian was reason enough to take his life. Kira, shaken and totally forlorn, whispers that it wasn't ...