“Necessary Evil” is episode eight of season two of Star Trek: Deep Space Nine.
Quark is hired by a Bajoran woman who lived on DS9 during the occupation and ran a shop there with her husband, and who he dealt with during that time, to recover something that was hidden in the shop. Quark being Quark, he decides to take a look at what it is. It’s a list, and he ends up getting shot for his troubles. Quark is seriously injured, but survives, thanks to Doctor Bashir.
The partner of the person in question was murdered, and this takes Odo back to his very first case. When the man was first killed, Gul Dukat brought Odo to the station and asked him to investigate the case, as Bajorans talk to Odo, but not, naturally enough, to Cardassians.
A suspect for the murder is a woman, recently arrived on the station, who was apparently having an affair with him – at least, according to the dead man’s widow. That woman is Kira, and this is the first meeting between Odo and her.
Whilst Odo investigates the present day attack, he also remembers events from the original crime. A crime which is, as yet, still unsolved. Although Odo believed the widow probably did it, there was no proof. Solving the new crime will probably solve the old one too, and Odo never closes an unsolved case.
This is the first episode in which Quark’s brother, Rom, starts showing intelligence and technical skill.