Harry Kim's romance with an alien leads to an infection.
Voyager helps repair the generational ship of the Varro, a reclusive people who have lived aboard it for four centuries and discourage contact with outsiders. Harry Kim begins a forbidden relationship with a Varro scientist named Tal, and their intimacy creates a biological bond that the Doctor treats as a medical condition, putting Kim at odds with Janeway.
At the same time, synthetic parasites spread damage through the Varro vessel. Tal turns out to be among the separatists who released them, hoping to split the ship so a minority could leave, and the sabotage threatens to tear it apart. Kim helps hold it together, the dissidents are allowed to depart, and he says a painful goodbye to Tal.