“By Inferno’s Light” is episode fifteen of season five of Star Trek: Deep Space Nine.
In the previous episode, “In Purgatory’s Shadow”, Worf and Garak had been captured by the Dominion and taken to an internment camp, where, amongst others, they discovered Dr. Bashir. He had been replaced on the station by a Changeling. Enabran Tain had also been revealed to be Garak’s father.
The Changeling had stopped DS9’s attempt to close the wormhole, and a fleet of Dominion ships had entered the Alpha Quadrant through the wormhole. The Jem Ha’dar don’t attack, but instead head to Cardassia – followed by Gul Dukat’s Bird of Prey. Dukat, who had been acting a bit suspiciously in retrospect, had been negotiating between the Dominion and the Cardassians, and Cardassia is now part of the Dominion, with Dukat now head of the Cardassian government.
With the Dominion in Cardassian space, the Klingons and Chancellor Gowron have been forced out and arrive at the station, asking for help. Sisko suggests that the treaty between the Klingon Empire and the Federation be restored, as DS9 helps fix the Klingon ships in anticipation of a Dominion/Cardassian attack. The Changeling Bashir is still undetected.
Back in the Gamma Quadrant, Garak, Worf, Dr. Bashir and General Martok are attempting to escape from the internment camp to the runabout, so that they can both escape and warn DS9 about the Changeling.