The crew's efforts to thwart a hijack scheme are complicated when a sinister alien criminal hides his consciousness within the brain of someone aboard the station.
Traveling in a runabout, Kira and Bashir pick up a distress signal from a disabled Kobliad ship. They beam over to discover an injured Kobliad woman named Ty Kajada, a security officer. Her passenger a murderer named Rao Vantika, who had set the ship on fire in order to escape. Vantika has been seriously hurt, and after attempting to strangle Bashir, falls over dead. Kajada collapses from her injuries. When she regains consciousness aboard the space station, she tells Bashir that Vantika has faked his demise before and asks the doctor to run tests to prove he is dead. Later, Bashir, Sisko, Dax, and Kira determine that Vantika was probably heading for Deep Space Nine to steal a shipment of deuridium, a rare substance that prolongs the dying Kobliad race's lifespan. Sisko orders Lieutenant George Primmin, a Starfleet security officer assigned to protect the deuridium, to defer to Odo's plan to guard that shipment when it arrives at the station. However, when Odo attempts to access his plan on the computer, it is gone. Everything in the station computer's active memory has been accessed and purged something Kajada grimly reports that Vantika has done before.
The computer mystery enables Kajada to convince Sisko assume that Vantika is still alive. That night, Quark also learns that Kobliad still lives when, hidden by darkness, Vantika orders the Ferengi to follow through on his promise to hire mercenaries to help him obtain the deuridium.
The next day, Bashir tells Kajada that his tests prove Vantika is really dead. He goes on to meet with Dax, who is exploring the idea that while Vantika's body is gone, his consciousness may have found a way to live on in another person's brain. The pair immediately suspect Kajada, deducing that Vantika could be occupying her brain without her knowledge while he waits for the deuridium. They share this theory with Sisko, and the group decides that Kajada must be watched. Meanwhile, at Quark's, the mercenaries arrive an alien named Durg and two Bajoran accomplices. Their haggling with Quark is interrupted by the sound of a woman screaming. Kajada, who was spying on the group, falls from the third floor balcony.
Now in the infirmary, Kajada reveals that she did not fall, but was actually pushed by Vantika. Meanwhile, Dax, who has been trying to discover how Vantika could transfer his consciousness into another person's brain, shows Sisko a small device buried under the dead Kobliad's fingernails. She believes Vantika stored his consciousness in the device as bio-electric pulses, in case he needed an escape route. As soon as Kajada's condition is stabilized, they can confirm the theory by examining her. Meanwhile, Quark, Durg, and the mercenaries prepare to meet Vantika in person and are shocked when he arrives in the physical form of Bashir.
Hoping to share her discovery with Bashir, Dax is surprised to find his combadge abandoned on a counter. Meanwhile, Primmin discovers that the station's defense array has been rigged to shut down which will allow Vantika to hijack the freighter carrying the deuridium. A grateful Odo informs Sisko of their discovery just as the freighter enters the vicinity. The group notices a runabout going out to meet the vessel just as Dax joins them with the announcement that Bashir is missing. Vantika and the mercenaries commandeer the freighter killing its crew. Suddenly, the hijacked ship is captured by a tractor beam from the space station. Sisko hails the freighter and speaks with Vantika, who threatens to destroy Bashir's body and the ship if he is not allowed to proceed. The ship's shields are up, so Bashir cannot be transported out. Dax is able to temporarily disrupt Vantika's control over Bashir by using an electromagnetic pulse to disrupt Vantika's neural energy patterns. She tells the dazed Bashir to lower the shields. He does, and the group transports him back aboard Deep Space Nine where he is purged of his evil possessor. A recovered Kajada is given Vantika's remains, and she destroys them, and her nightmare, with a single phaser blast.
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