Did we skip something? Kira and Bashir are discussing some medical miracle Bashir just performed. Then they receive a distress call and turn the runabout to go investigate. They beam aboard the stricken ship where, against one of the survivor's wishes, Bashir unseals a prison cell to help the occupant. Although the occupant has enough strength to try to throttle Bashir, he expires.
Back on DS9, Bashir treats sole survivor Ty Kajada, who isn't convinced the dead prisoner is actually dead. She stabs the body a couple of times, just to make sure.
Meanwhile, Odo and Quark exchange banter, this time over whether Dax fancies Quark. This quickly veers into TMI territory. Odo appears to have peeved a Starfleet security officer because he mentioned a shipment to Quark. Lt Primmin goes to complain to Sisko about it and promptly gets bitched out. He goes to apologise to Odo, and the discover that the station computer's been wiped. Kajada reports this is something the dead criminal (Rao Vantika) likes to do. They set about trying to find out where the undead Vantika is, then Odo expresses doubts about Primmin.
Meanwhile, Quark is "cleaning" or searching for lost jewelery, when he's jumped by someone who appears to be Vantika and wants to know why Quark hasn't held up his end of their agreement. I bet he gets that a lot.
Bashir says the DNA results say the body is definitely that of Rao Vantika. Dax thinks that Vantika may have transferred his mind into another brain. They suspect Kajada, so Odo restricts security access to four people. Kajada is miffed, but thinks Quark might be associated with Vantika, so drops in (literally) on one of his deals.
In sick bay, Kajada claims she was pushed. Dax is working on a theory, and finds a microscopic device under dead Vantika's fingernails - the transfer mechanism that he must have used to transfer his mind. Meanwhile, Odo and Kira have lost Primmin.
Quark and his cohorts find Bashir in a runabout - Vantika is hiding in Bashir's head. Odo locates Primmin, who's found a glitch in the security backups which would have taken down the whole security system for an hour. The freighter with the deuridium on board appears through the wormhole, and Bashir meets it in the runabout.
The Cohorts board the freighter and kill the crew. Unfortunately for them, DS9 locks on a tractor beam. Apparently being possessed by Vantika involved talking very slowly and losing all acting ability. Sisko is appalled. Dax works out a technobabble EMP to give Bashir back control of his body. Surprisingly, the plan actually works, and they retrieve him from the ship.
Dax then works out a way to beam the bits of Bashir's brain with Vantika in into a petri dish. What?! surprising this also works, and Bashir wakes up with a headache.
After checking that Sisko's returned custody of the prisoner to her, Kajada disintegrates the petri dish. Way harsh! Teh end.