A Trill candidate for joining with a symbiont arrives on DS9 for observation by Dax, who will report his psychological fitness to the Symbiosis Commission on the Trill homeworld. In the course of their bumpy relationship - based on the fact that the candidate prepared himself to be in the company of Curzon Dax and not Jadzia - they pick up something unusual from the wormhole and bring it back to the station for study, not realizing that it will begin expanding rapidly until it threatens to consume the station...and the space beyond.
