He reveals 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.
With reinforcements at least two days away and only disgraced Cardassian officer Dukat (on the station due to a confrontation with a Klingon battle cruiser) 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.
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.
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.
However, the beam fails due to the emitter array having been sabotaged and the wormhole remains open, allowing a fleet of Dominion warships to begin pouring through.
He found some of the setup and character scenes dragged and were a little frustrating at times, but appreciated how the episode brought things into greater focus.
[4] In 2018, CBR rated "In Purgatory's Shadow" paired with "By Inferno's Light", as the 9th best multi-episode story arc of all of Star Trek.