Major Kira and Doctor Bashir are returning from a medical mission in the runabout USS Rio Grande. Kira was reluctantly impressed by Bashir’s competence after he found a vital clue in a woman whom she dismissed as deceased. Bashir appears to accept such praise as no more than his due, and Kira is about to tell him off for his arrogance, when they receive a distress call from the Reyab, a Kobliad transport. They beam aboard to find that the ship’s systems have been disabled by a fire raging on board.
Kira looks for fire safety gear, and Bashir goes to the nearest crewperson, a Kobliad female named Ty Kajada. He asks her if anyone else is on board, and she says the pilot is dead. Bashir reads another lifeform behind a locked door from his tricorder. Kajada warns for him not to open the door; that the man is a prisoner who started the fire in an attempt to escape. Over her protests, Bashir overrides the door lock and finds a dying Kobliad male, Rao Vantika. Kira comes back and extinguishes the fire, preparing all four of them for emergency transport back to the runabout. Before she can energize, Vantika seizes Bashir by the throat with an iron grip and pulls him close. As Kira tries to pry his hand loose (she didn’t try that hard now, did she ? ^^), Vantika growls, “make me live“, before expiring. “This one I can’t bring back,” Bashir muses over Vantika’s lifeless body.
Back on Deep Space 9, Bashir revives Kajada in the infirmary with a hypospray. When she awakes, her thoughts go immediately to her prisoner. Bashir says the man is dead, but Kajada insists on examining his body herself. Bashir, a little insulted, assures her that Vantika is quite dead. She retorts that she has tracked Vantika, a multiple murderer, for over twenty years, during which he has faked his death more times than she can count, and more than a few medical “experts” have been fooled. After scanning Vantika’s retinas to confirm that he is brain dead, Kajada shocks Bashir by coolly stabbing the corpse in the chest with a trocar. She asks Bashir to autopsy the body and confirm its identity with DNA records, and to scan her ship for anomalous life readings. Shaken, Bashir agrees.
In Quark’s, Quark delivers a drink with unctuous good wishes to Jadzia Dax, while Odo scornfully asks what he thinks his chances are with a woman like her. Quark retorts that nothing ventured, nothing gained, and not everyone in the galaxy is comfortable being as lonely and miserable as Odo. Odo is then put out to receive a visitor, Starfleet Security officer Lieutenant Primmin, who says they have been assigned to work together.
In Ops, Bashir reports that every test he has run confirms that the dead Kobliad is Rao Vantika, and that he is indisputably dead. Kajada has warned that Vantika was on his way to Deep Space 9 when she captured him, and Sisko and Dax connect his destination with a shipment of deuridium ore scheduled to arrive at the station from the Gamma Quadrant. The Kobliads are a dying species, and need deuridium to stabilize their cell structures. Even with new deposits discovered in the Gamma Quadrant, the demand far outstrips the supply, so it makes sense that Vantika would want to steal a supply for himself.
Bashir says Kajada still isn’t convinced Vantika is dead, so Sisko agrees to her request to scan her ship. He is also concerned with unearthing any preparations or accomplices Vantika had on the station in preparation for the hijack.
In conference with Primmin, Sisko quickly disabuses him of his belief that the station’s security needs an “overhaul” to conform with Starfleet Security regulations; Odo has his own way of doing things, and Sisko has found that way to be effective, and best suited to the unique situations aboard the station. Sisko encourages Primmin to be a little more flexible, as he could actually learn something from “The Constable;” Primmin assures Sisko that he has made himself very clear.
With some difficulty, Odo and Primmin begin working together, reviewing the security arrangements for the deuridium shipment. When Odo tries to access the computer, however, he is surprised to find that everything in the active memory banks has been accessed and then purged. He believes this is impossible, but Kajada appears in his office and says that Vantika has done exactly the same thing before on Rigel VII.
In Ops, Sisko and his crew confer, with Kajada and Primmin present. Kajada says that Vantika usually pulls this sabotage trick by planting a subspace shunt in a nonessential system that would be overlooked by security checks. Odo concedes it is an elegant trick: rather than try to get past security programs guarding a particular file, a criminal simply performs a dump of the whole memory, which has not or cannot be guarded against. When a scan of the station detects an anomalous device planted, Odo (and Primmin) both order a security patrol to retrieve it.
During these instructions, Kajada constantly speaks as though Vantika has done these things himself, but Sisko remains skeptical, and asks her whether it is more likely that some accomplice of Vantika is responsible. Kajada assures them all that Vantika wouldn’t have started the fire on board the ship unless he had a plan for living through it; she doesn’t know what that plan was, but the sabotage aboard the station is enough to convince her that it succeeded. Impressed by her arguments, Sisko instructs the crew to assume for the time being that Vantika is alive, until Bashir receives DNA confirmation from Kobliad Security of the corpse’s identity.
Odo asks to have a word with Sisko in his office. He begins to complain about Primmin’s presence, but Sisko already knows what he’ll say. He reminds Odo that Starfleet has its own interests on the station, which makes Primmin a fact of life that Odo will have to deal with. Sisko says he has already encouraged Primmin to work with Odo, but Odo isn’t satisfied. He wants to know who’s in charge, and Sisko says he is. Trying not to look too pleased, Odo admits, “I can live with that,” and departs.
In Dax’s lab, she reports to Sisko that someone tried to break into the cargo hold of the Reyab after it was docked at Deep Space 9. What they could have been looking for, Dax reports, is an isolinear chip she found, containing a “map” of the humanoid brain.
In Quark’s Bar, Quark is on his hands and knees collecting his “tips” (dropped money) from the floor of the upper level, when someone seizes him from behind and whispers in his ear, asking him whether he’s made the preparations for the deuridium shipment – hiring a group of mercenaries. Baffled, Quark says that he has done so, but he was told by reputable sources that Vantika was dead. The voice whispers, “almost, but not quite,” then releases Quark and retreats into the shadows.
Bashir calls Kajada into the infirmary and triumphantly shows her the final results of the DNA comparison; every conceivable test has been run and confirms that the Kobliad in the morgue is dead, and that he is Rao Vantika. To his amazement, Kajada still isn’t convinced.
Dax calls Bashir into her lab and shows him Vantika’s computer models. She theorizes that Vantika was developing a means of transferring his consciousness into another’s brain. Bashir says that such a thing is theoretically possible, and if Vantika succeeded, then his most likely “host” is Kajada, the nearest available Kobliad.
They present their theory to Sisko, who isn’t convinced. Why would Kajada be warning them if Vantika was controlling her? Bashir explains that, according to their theory, Vantika is only controlling Kajada part of the time; the rest of the time, he’s simply “along for the ride,” and Kajada isn’t even aware of his presence. Sisko asks for proof, and Dax says that they can’t know what to look for until they have a better idea how Vantika accomplished the transfer.
Odo is in his office when Kajada storms in, protesting that he’s revoked her security access, and is refusing to answer her questions about the precautions for the deuridium shipment. He says he is just being extra cautious, and she storms out.
Suspecting that Quark may be assisting Vantika, Kajada spies on him from the upper level of the bar after closing time. She finds him meeting with the mercenaries he summoned. Suddenly, they look up when they hear a scream, and see Kajada, hanging from the balcony rail. Before any of them can react, she loses her grip and plummets to the deck.
Kajada lies in a coma in the infirmary, but not before saying she was pushed by Vantika. Bashir reports to Sisko that he cannot examine her more closely until she regains consciousness. Sisko theorizes that Kajada may have tried to kill herself after realizing she was carrying Vantika around with her.
In the morgue, Sisko finds Dax taking samples from under Vantika’s fingernails. She says she thinks she has found his transfer method: a biocoded message imprinted onto a glial cell that allows a message to be transmitted along a humanoid’s nervous system directly to the brain. She found models of the glial cells on Vantika’s data chip. The most effective means of transfer would be to inject the cells directly into the victim’s skin, but there were no needles or hyposprays in Vantika’s belongings. Examining the tissue from under his nails, Dax finds a microscopic electric generator, to “fire” the message along the nervous system. Vantika hid the device there, just to give himself a last-ditch escape.
In the docking ring, Quark leads the three mercenaries to one of the runabout pads, explaining that their mysterious employer managed to access one and is waiting for them on board. The door opens, and there is Dr. Bashir. Quark backpedals, stammering that they took a wrong turn. But “Bashir” steps forward, smiles coldly, and says, in a voice quite unlike his own, that on the contrary, he has been expecting them.
Dax enters the infirmary, calling for Bashir. He does not answer. She asks the computer to locate him, and it answers “the infirmary.” She finds his combadge left on a counter.
Odo meets with Kira’s team and is surprised to find that Primmin is not working alongside them as agreed. When Odo finds Primmin, the lieutenant explains that he decided to take a hint from Odo’s methods, and tried to think like Vantika. Doing so has led him to discover a second subspace shunt hidden in the waste extraction system. Impressed, Odo says that the device, if left in place, would have disabled the station for close to an hour.
As the deuridium cargo ship comes through the wormhole, a runabout leaves the station, unscheduled, to escort it back to the station. In Ops, the crew discover that Doctor Bashir is missing, and that it was his access codes that enabled the launch of the runabout.
On the cargo ship, the three mercenaries beam aboard and kill the bridge crew, before signaling Vantika/Bashir that it’s safe to board. Sending two of the mercenaries to secure the rest of the ship, Vantika orders the shields raised and a new course layed in. Before the ship can engage, the station locks a tractor beam on them. Vantika is frustrated, because he thought he’d sabotaged them. Sisko hails the ship, and Vantika introduces himself from Bashir’s body. He demands that the tractor beam be removed, or else he’ll engage the warp drive, tearing the ship to pieces – killing Bashir and the remaining crew and spreading deuridium (a hazardous material) all over the system. When the mercenary on the bridge refuses to commit suicide, Vantika shoots him dead with his phaser. He gives Sisko one minute to decide, then cuts off the transmission.
Sisko asks for suggestions. The ship’s shields are raised, and any attack to overwhelm them would risk breaching the ship’s hull. Dax comes up with a way of sending an electromagnetic pulse along the tractor beam, where it will resonate off the shields and cause an energy field inside the ship that will disrupt Vantika’s hold over Bashir. It is successful, long enough for Sisko to get through to a very confused Bashir. The doctor lowers the ship’s shields, and is beamed to Ops. As Rao tries to seize control of him again, Sisko stuns him with a phaser, and they take him to Dax’s lab. With a specially programmed transporter, she removes Vantika’s signature glial cells and transfers them to a small containment field. Scanning the unconscious Bashir, Dax pronounces him “clear,” and revives him. Waking up with a groan, Bashir’s first words are to complain of a splitting headache.
A short time later, Sisko, Bashir, Dax, and a recovered Kajada regard the containment field in Dax’s lab. Kajada apologizes to Bashir for his ordeal, while Bashir feels that they all owe her an apology for not taking her warnings about Vantika more seriously. Bashir confesses that he doesn’t remember any of what Vantika made him do, and feels humiliated for not being able to stop it. Sisko assures him that no one holds him responsible.
Kajada asks Sisko to formally return custody of “the prisoner” to her. As soon as he agrees, Kajada draws her phaser and disintegrates the container, wiping out the last traces of Vantika, then walks out without another word.