When Voyager encounters an alien ship they have never seen before, the Nasari, Ensign Kim instinctively fires Voyager's weapons upon it without orders, claiming that a tetryon surge was a manner of charging weapons. Voyager takes heavy damage in defeating this enemy and making them withdraw. Kim is relieved of duty, pending investigation of his overtly hostile actions.
During the night, he has a strange dream, and when he awakens, he has a rash on his head. The Doctor cannot find an initial cause. But when Harry's instincts also tell him to flee to a nearby planet at the next attack by the Nasari, they are defended and then hailed by the Taresians.
Captain's log, stardate 50732.4. The Taresians have escorted us back to their home world so we can continue to investigate their claim that Ensign Kim is a member of their race.
They explain a shocking story of how Harry was implanted with their DNA at birth, as the allure of the mostly female population and a new identity excite Harry. Voyager leaves soon to try and negotiate with the Nasari, and Harry learns more and more about their culture. He learns that he is breeding stock, but is so rare that he will be an honored guest.
It is only now that the doctor discovers something about his DNA: it was implanted, possibly by a virus. The Captain also realizes that the defensive grid they raised after Voyager left blocks communications, making it easy to believe they don't want him to leave.
After spending a night there, and having more strange dreams, doubt begins to creep up on him. Two very "interested" females try to reassure him, but their attempts to do so make him outright suspicious, and eventually reveal what the doctor has discovered. His attempt to escape reveals an even more disturbing truth: the reproductive process is fatal to the male, meaning that more males have to be repeatedly harvested from outer space.
Just in time, Voyager gets inside the grid and beams Harry up. When they leave the system, the Nasaris following Voyager attack the Taresians instead.