At a time when Deep Space Nine is all but empty, an unjoined Trill and his crew, a pair of hired Klingon mercenaries and a former prostitute named Mareel, overpower the crew of the station. The Trill, Verad, applied to be "joined" and was rejected; now he wants the Dax symbiont. After restraining the shapeshiftingConstable 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 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 remains loyal to him.
Quark, who is responsible for Verad and his crew boarding the station, 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 realizes 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. Major Kira overpowers the Klingon who is holding her, but in the confusion Verad slips away, heading for another runabout pad. In the mean time, Mareel has realized that the man she loves truly is gone and decides to help Sisko. Sisko confronts Verad at the airlock to the runabout. Believing that Sisko will not shoot his old friend and risk damaging the recently joined symbiont, Verad begins to walk away, but Sisko shoots him, declaring, "Don't call me Benjamin!" The Dax symbiont is returned to Jadzia, leaving Verad alone with himself once again.