When an Away Team consisting of Paris, Torres and Lt. Durst fails to return, Chakotay, Kim and Tuvok beam down to the planet where the trio was working to investigate. Chakotay traces the missing crewmembers to some caves, but can't break through the Vidiian force field to find them.
In an underground lab, a Vidiian scientist named Sulan has extracted Torres's Klingon genetic material to create an all-Klingon version of her. He hopes to create a cure for the deadly Phage disease infecting his race by injecting a pure Klingon subject with the disease. But his interest in Torres is more than purely scientific, a fact the Klingon woman soon picks up on.
Trapped in another Vidiian cell, Paris and Durst are stunned when a new prisoner is brought in. It's an all-human version of Torres — the other "half" of Torres that was left after Sulan removed the Klingon genetic material. The guards return and forcibly remove Durst from the cell. Although Paris tries unsuccessfully to help his friend, all the human Torres can do is cringe in fear.
The next time Sulan comes to visit the Klingon Torres, she is horrified to see him wearing Durst's face. He has killed the lieutenant and grafted the dead man's face over his own diseased features in the hopes that Torres will find him more attractive. Instead, her anger helps her to break the bonds restraining her. She attacks Sulan and escapes from the lab.
Paris and the human Torres are sent out on a labor detail, but when she can no longer work, she's sent back to the barracks. Her Klingon counterpart finds her there, and helps her to escape. In turn, the human Torres comes up with a plan to deactivate the shields for the whole facility, which would allow Voyager to beam them up. The two beings acknowledge that each has unique qualities that contribute to the whole being.
Disguised as a Vidiian, Chakotay rescues Paris and they reunite with the two Torreses. Just as they are about to beam back up to the ship, Sulan fires and the Klingon Torres takes the hit to save her human self. She dies after beaming up with the others, but the Doctor is able to use her Klingon DNA to restore Torres back to her original self.