A violent plasma storm leads to the evacuation of Deep Space Nine, leaving a skeleton crew behind to maintain the station. Odo is suspicious when he learns that Quark is hiding in an airlock, but can pin nothing on the Ferengi. The crew picks up a distress signal from a cargo ship that has been damaged and needs assistance. O'Brien and Odo meet the ship at a docking bay, where the "victims," a humanoid female named Mareel, Klingon mercenaries T'Kar and Yeto, and a nervous male Trill named Verad, pull weapons on the pair and force Odo to assume liquid form and get into a small box. The group then forces Bashir to put Odo in stasis, and finally winds up in Ops, taking everyone hostage. O'Brien deduces that Quark is connected to their plight, unaware that, at that very moment, Yeto is revealing to the Ferengi that, what Quark thought was commerce, was really a trick to get the group aboard. T'Kar reveals the reason for the takeover — the timid Verad is actually the leader and has come to steal Jadzia's symbiont.
Unable to look Dax in the eyes, Verad says he has spent his whole life trying to qualify for symbiosis, but has been deemed an unacceptable candidate. Tired of his mediocre life, he has decided to take Dax's symbiont and escape into the Gamma Quadrant. Dax urges him to reconsider, reminding him that most Trills are not chosen for symbiosis, and that improper joining can cause psychological damage to both symbiont and host. But Verad threatens to kill the rest of the crew if Dax refuses, and she agrees to go through with the procedure that will surely kill her. Mareel gives Verad a tender kiss goodbye, and Bashir reluctantly performs the operation, solemnly removing the symbiont. Back in Ops, Mareel tells Kira about her love and devotion to Verad when Sisko jumps in, warning her that Verad is sure to be greatly changed after the surgery, becoming a mixture of Verad, Dax and all of Dax's previous hosts. Mareel replies that she will always love him, just as a far more confident Verad walks in, but he is now Verad Dax.
While Bashir desperately tries to hold on to Jadzia's life, Sisko talks with Verad, tapping into the lifelong friendship he and the Dax symbiont have shared. Verad reminisces about old times until the memories begin to involve Jadzia, who, Sisko reminds him, is currently dying because of Verad. Sisko urges Verad to give the symbiont back to Jadzia, but Verad is unwilling to part with it, explaining that another joining so soon may destroy the symbiont. Sisko, willing to take that risk, walks out, saying that their friendship is over.
Unable to convince Verad Dax, Sisko turns his attention back to Mareel, urging her to talk with Verad and see for herself how much he has changed. She refuses, claiming to want to let him rest, but Sisko's words are getting to her. Meanwhile, Quark, trying to atone for what he's done, jumps T'Kar and is immediately injured. Verad Dax and Mareel make their plans to escape, and Verad tells her he will leave first and she is to meet him at a designated rendezvous point. Despite his assurances, Mareel is hesitant, realizing that Verad has, in fact, changed. Back in surgery, Bashir knocks Yeto unconscious with a hypospray and then he and Quark release Odo from stasis. Soon afterward, Verad attempts to contact Yeto and realizes that Odo must have been freed. He decides to leave immediately with T'Kar, taking Kira along as a hostage and leaving the dying Jadzia behind.
With Verad Dax gone, Sisko turns back to Mareel, who reveals that Verad has lied to her for the first time and that she fears he will not meet her as planned. Sisko takes advantage of her sadness, reminding her that the old Verad still cares about her, and that by saving Jadzia, Verad will be saved as well. Meanwhile, Verad arrives in the airlock to discover that his ship is not there. Odo suddenly appears, morphing out of a tool cart, and tells him that he released the docking clamps. Kira then attacks T'Kar and a struggle ensues, but Verad escapes to a Runabout airlock just as Sisko appears and pulls a phaser on him. Verad tells Sisko he doubts his old friend will fire and risk killing the Dax symbiont. But Sisko does fire, and Verad soon wakes up in surgery. Although Mareel professes her undying love for him, he is inconsolable over the loss of the symbiont. Dax, however, is back to normal, with the addition of all of Verad's memories and feelings.