"Necessary Evil" is the 28th episode of the American science fiction television series Star Trek: Deep Space Nine. It is the eighth episode of the second season. Set in the 24th century, the series follows the adventures on Deep Space Nine, a space station near the planet Bajor, as the Bajorans recover from a long, brutal occupation by the imperialistic Cardassians. This episode focuses on Deep Space Nine's security chief Odo, a shapeshifter of unknown origin. In this episode, an attack on the bartender Quark leads Odo to re-open an investigation into an unsolved murder dating back to the days of the Cardassian Occupation. The episode features flashbacks to that time period, including the first time Odo met the Bajoran resistance fighter Kira Nerys, who is Deep Space Nine's first officer during the time period in which the series is set. Plot A Bajoran woman, Vaatrik Pallra, hires Quark to retrieve a strongbox from her late husband's shop on Deep Space Nine. Quark opens it to find a list of Bajoran names. A stranger, Trazko, sneaks up, shoots Quark and steals the list. As Dr. Bashir tries to revive Quark, Quark's brother Rom tells Odo about the list of names, and that the box was hidden in the shop during the Occupation. In a flashback, five years earlier, Odo is solicited by the station's Cardassian commander Gul Dukat to investigate a murder. Odo interrogates Vaatrik Pallra, the deceased's widow. She claims her husband had been having an affair with Kira Nerys. When he interviews Kira, she says that there was no intimate relationship between her and Vaatrik, and that she was interviewing for a job at Quark's bar at the time of the murder. In the present, Rom tells Odo that one name on the list resembled "Ches'so". Odo interviews Pallra, who claims ignorance of the list of names and the name "Ches'so". She also refuses to divulge who loaned her the money to pay her past-due electricity bill. Kira identifies "Ches'so": a philanthropist named "Ches'sarro", who has just died. Five years earlier, Quark admits that Kira bribed him to provide a false alibi. When Odo confronts her about it, Kira reveals that she was committing a terrorist attack on the station. Odo tells Dukat that Kira is not the culprit in the murder. In the present, Odo deduces that the names on the list are Bajorans who collaborated with