“Invasive Procedures” is episode four of season two of Star Trek: Deep Space Nine.
DS9 is running on a skeleton crew – basically, the command staff and Quark – and is largely powered down due to a plasma storm, when a freighter hails them asking for help. The ship is towed into dock, but when O’Brien and Odo go and greet it, four armed people come out.
One of the four, the leader, who has hired two Klingon mercenaries to help him, is a Trill, like Jadzia. Unlike Jadzia, he hasn’t been joined with a symbiont. Although he trained for it, when he went in front of the review board, he was rejected as being unsuitable. So he’s decided to take matters into his own hands by stealing a symbiont – Dax. Which kind of underlines the whole ‘unsuitable’ bit.
Once the symbiont is removed, Jadzia will likely die within a matter of hours, so Sisko is going to have to do something about it.