"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 d