Both Tuvok and Chakotay return from a routine expedition injured -- Tuvok with minor ailments and Chakotay greviously injured. During an encounter with a black nebula, Chakotay's neural energy was depleted; unless it is returned, Chakotay will be doomed to a life as a brain-dead host. While trying to return to the nebula, the crew of Voyager discovers that an alien presence is aboard and able to control their efforts.