The crew is attacked by a disembodied consciousness.
Chakotay and Tuvok return from a dark matter nebula in a damaged shuttle, with Tuvok unconscious and Chakotay apparently brain-dead. As Voyager heads back toward the nebula, crew members begin changing the ship's course and systems with no memory of doing so, and it becomes clear that an unseen presence is moving from body to body.
The culprit turns out to be Tuvok, possessed by a Komar, a noncorporeal being that wants to lure the crew to the nebula's heart and feed on their neural energy. A second presence aboard has been working against it: Chakotay's own displaced consciousness. Once the Komar is driven out, Chakotay helps guide the ship out of the nebula and is restored to his body.