Quark is recruited to retrieve a list of names from a strongbox on DS9.
'Necessary Evil' is a strong episode on a number of levels.
The central mystery is pretty compelling and is another instance of DS9 depicting the darker and more complex side of characters and events. It unfolds in a mostly entertaining way throughout, however the final reveal is portrayed in a fairly standard confession dialogue.
Great moments for Odo, Kira and Gul Dukat are another positive. These are some of the most interesting characters on DS9, played by the most charismatic performers. Seeing them on DS9 under Cardassian rule adds another level of intrigue.
Visually it is one of the best episodes of the Star Trek franchise. The cinematography, colours, and lighting give so many scenes a superb atmosphere and work so well for the film noir tribute. Also the editing works seamlessly for the time jumps.
Rene Auberjonois leads it fantastically well with great support from the likes of Nana Visitor, Marc Alaimo, and Armin Shimerman.
For me it's an 8.5/10 but I round upwards.