DS9 Season 5 Episode 14 In Purgatory’s Shadow

Plot

When Deep Space Nine picks up a mysterious coded message from inside the Gamma Quadrant that appears to be Cardassian, exiled Cardassian spy Garak is asked to analyze it. He determines that the message is a distress call from his mentor Enabran Tain, who was believed to have been killed in an ill-fated Cardassian attack on the Dominion two years prior. Garak convinces Captain Sisko to allow him to travel to the Gamma Quadrant, accompanied by Lt. Cmdr. Worf, to search for Tain and other possible survivors from Dominion attacks.
Soon after entering Dominion space, they wind up in the midst of the Dominion fleet. Worf realizes that the fleet must be assembled to invade the Alpha Quadrant. He sends a warning message to DS9 just before Jem'Hadar soldiers take him and Garak prisoner. Back on DS9, the crew receives Worf's message and realize the Dominion is coming. With reinforcements at least two days away and only disgraced Cardassian officer Dukat on hand to help out, Sisko sees only one choice—sealing the wormhole, trapping Worf and Garak on the other side if they cannot return before the operation is completed.
Worf and Garak are taken to a Dominion detention center. Tain is there, on his deathbed; instead of being thankful to Garak for coming, Tain chastises him for allowing himself to be taken prisoner. Also among the fellow prisoners are the Klingon general Martok, and, to Garak and Worf's surprise, Dr. Julian Bashir—meaning that the Bashir back at DS9 is a Changeling impostor.
As his last request, Tain makes Garak promise to escape. Desperate for a kind word from his mentor before the old man passes on, Garak makes a request in return: that Tain acknowledge him as his son. Tain does so, and father and son share a memory before Tain dies peacefully. His mission complete, Garak is ready to find a way out.
On Deep Space Nine, the crew shoots a particle beam at the wormhole, intending to close it. However, something goes wrong. The emitter array has been sabotaged, and the wormhole remains open, allowing a fleet of Dominion warships to begin pouring through.