A severe plasma storm leads DS9 to be temporarily but completely evacuated, save the senior officers (of course) who remain on board. After the evacuation, an unjoined Trill named Verad (John Glover) and his small band of mercenaries gain access to the abandoned station and hold the officers hostage. Verad has come to steal the Dax symbiont from Jadzia—which he believes is rightly his—and damn the consequences that Jadzia will die within hours of the operation.
"Invasive Procedures" is an engaging character story with some surprising depth—though it also has some evident flaws. The premise that sets the episode in motion—the convenient abandoning of DS9—is totally contrived. And the idea that Quark bypasses station security and unwittingly allows these mercenaries to board the station purports to have dire consequences ("You're through here," Kira promises at one point), but ultimately comes off awfully transparent.
But the core of this episode is very strong, featuring some excellent characterizations. John Glover's portrayal of an inept, confidence-lacking Verad is brilliant. Equally brilliant is his change of personality once he steals the symbiont and joins with it, becoming an interesting blend of Verad and Dax. Sisko's response to the joining is utterly fascinating to watch, as he attempts to reason with the friend he knows resides inside the new Verad. Sisko also slowly wears down Verad's girlfriend Mareel (Megan Gallagher), who watches in dismay as the man she knows transforms into a personality who subtly reveals he doesn't "need" her anymore.
The subplotting involving Bashir, Quark, and the Klingon mercenaries (Steve Rankin and Voyager's Tim Russ) proves amusing, albeit routine. But the psychological twists and turns involving Sisko, Mareel, and the tragic Verad keeps this show in top form.