An attempt on Quark's life sheds new light on a murder investigation from years ago that Odo had never solved, and as he looks at the new evidence (a mysterious list of names) he assembles the clues from the past and present to uncover the killer of a Bajoran chemist.
One real strength to "Necessary Evil" is its remarkably compelling flashback sequences, which are woven into the story flawlessly. James Conway's stellar direction and the standout art design and production takes us back to Terok Nor of five years earlier, creating a dark, malevolent slave mining station run by Gul Dukat. The lighting and photography is nothing short of brilliant—creating a true Trek noir—but the characterizations and story events are just as powerfully drawn.
Odo's investigating techniques highlight his intelligence, patience, and thoroughness extremely well—and his pointed commentary about justice in both the dialog and the running security log voice-overs highlights many keen observations. The flashbacks plausibly and interestingly document the way Odo met Kira, Dukat, and Quark all within the same investigation. The story's use of the murdered man's widow, Pallra (Katherine Moffat), is also nicely realized.
Kira's role in the investigation is especially intriguing, opening the door to more dark chapters of the freedom fighter's violent past. The revelation that she was actually the killer—on assignment by the Bajoran underground—is gutsy and probing, weakening a bond of trust shared between her and Odo. With a plot that is beautifully crafted, its powerful and thoughtful dialog, and a multitude of riveting character implications, "Necessary Evil" is one of the series' all-time best installments.