Plot summary
During a plasma storm, Deep Space Nine is evacuated, with only a skeleton crew remaining behind to maintain the station until the event passes. Posing as a cargo transport in distress, an unjoined Trill, a pair of hired Klingon mercenaries, and a former prostitute named Mareel board the station and overpower the crew. Major Kira tries to take Mareel by surprise, but Mareel outfights her. The Trill, Verad, once applied to be joined with a symbiont and was rejected; now he wants the Dax symbiont. After restraining the shapeshifting Constable Odo in a container and taking the rest of the crew hostage, he forces Dr. Bashir to transfer the symbiont into his body. Without her symbiont, Jadzia will die within hours.
Bashir has one of the Klingons assist him as he tries to save Jadzia's life. Meanwhile, Commander Sisko deals with Verad, who now has the memories of all the Dax hosts, including Curzon Dax, who was a very close friend and mentor to Sisko. He implores Verad to set things right, but sees that he intends to let Jadzia die. Meanwhile, Mareel, whom Verad had befriended early in life, begins to realize how much the man has changed since he received the symbiont, and begins to think the joining may have been a bad idea. However, she remains loyal to him.
The bartender Quark, who helped Verad and his crew board the station by bypassing the security lockdown, pretends to be injured and thereby provides Bashir with the opportunity to sedate one of the Klingons. He then cracks the lock on the container holding Odo. Once Verad discovers what has happened, he heads for his ship, taking Kira as a hostage.
By the time Verad reaches his ship, Odo has released the docking clamps, leaving him stranded. Kira overpowers the Klingon who is holding her, but in the confusion Verad slips away, heading for another ship. In the meantime, Mareel now knows that the man she loves truly is gone and decides to help Sisko. Sisko confronts Verad at the airlock. Believing that Sisko will not shoot his old friend and risk damaging the recently joined symbiont, Verad begins to walk away, but Sisko stuns him with a phaser. The Dax symbiont is returned to Jadzia, leaving Verad alone once again.
Credits
Adapted from Wikipedia contributors, Invasive Procedures (Star Trek: Deep Space Nine). Paragraphs and formatting adjusted; illustrations, reference markers, block quotations and non-story material omitted.