At a time when Deep Space Nine is all but empty, an unjoined Trill and his crew, a pair of Klingons and a former prostitute named Mareel, overpower the crew of the station. Mareel becomes one of the few people to ever beat Major Kira in single hand-to-hand combat. The Trill, Verad, applied to be "joined" and was rejected; now he wants the Dax symbiont. After restraining 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, 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 the Trill to set things right, but he sees that Verad intends to let Jadzia die. Meanwhile, Mareel, who 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 retains her loyalty to him.
Quark, who is responsible for Verad and his crew boarding the station, pretends to be ill so Bashir can knock one of the Klingons out. He then cracks the lock on the container Odo is in. Once Verad realizes what has happened, he heads for his ship and takes 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 a runabout. In the mean time, Mareel has realized that the man she loves truly is gone and decided to help Sisko. Sisko meets Verad at the airlock to the runabout and the two have a stand off. Believing that Sisko will not shoot his old friend, Verad begins to walk away, but Sisko shoots him, declaring, "Don't call me Benjamin." The Dax symbiont is transferred back to Jadzia, leaving Verad alone with himself once again.