Sisko, Kira, and Bashir take Kai Opaka (Camille Saviola) through the wormhole for a brief excursion, but their Runabout is shot down by an automated satellite system when investigating a nearby planet. Opaka is killed in the crash in a surprising, tragic twist of fate. Meanwhile, the survivors find themselves in the middle of two warring factions of a penal colony. The twist: no one on this colony dies; their cells have been re-engineered to keep them fighting a futile battle with no possible victory for either side.
Once you accept the implausibility of immortality in this sort of warfare (couldn't you sever your enemy's head to kill him?), this story's premise is intriguing. Ambitious production design stands out, as do some above-average fight scenes. The episode's relevant theme is the prisoners' error of continuing the pointless violence; neither faction can put aside their hate and think their situation through. When the same condition that gives the prisoners their immortality brings Opaka back from the dead, the episode wisely uses her character as a symbol of non-violence.
The most interesting character aspect is the focus on Kira's violent past and how, with the Kai's help, she comes to realize this internal conflict and decides to begin the journey of leaving her violent impulses behind. Opaka's subsequent decision to remain among the prisoners with the hope of helping them end their conflict and beginning their healing uses the foreshadowed idea of "destiny" rather well. It shows a character who has a new mission ahead of her, and it also promises to bring about sweeping changes on Bajor as a result of the important figure's absence. Running alongside the Kira storyline, this makes a lot of sense. The final line of dialog hints that we may see a follow-up to the storyline, which I definitely would welcome (even if it turns out to be five years down the road).