The Voyager crew recovers its shuttlecraft containing Commander Chakotay and Chief of Security Tuvok, who had been exploring a nearby dark matter nebula. Tuvok is unconscious, but Chakotay appears to be brain dead, lacking any neural activity. He is put on life support by the Doctor. The shuttlecraft shows evidence of energy weapon attacks. Captain Kathryn Janeway orders Voyager towards the nebula to investigate. B'Elanna Torres, a close friend of Chakotay, uses a Healing Wheel, one of Chakotay's spiritual talismans, hoping to guide Chakotay's spirit to his body.

En route to the nebula, the ship suddenly changes course away from it, though Lieutenant Tom Paris, at the helm, denies making the change. When it happens a second time, apparently at Paris's instigation, he is relieved of duty until the cause of the course changes can be determined. As the crew follows a complicated route through the nebula, the ship's engines are suddenly deactivated, apparently by Torres, who like Paris also has no recollection of doing so. The Doctor discovers a common memory pattern between Paris and Torres at the times they acted, indicating that they were possessed by another entity. Kes states that she has felt the presence of such an entity, and Tuvok offers to perform a mind meld to try to help her identify it. However, the two are soon found unconscious, with Kes in a coma. Unsure of the intentions of the entity, Janeway orders the command codes transferred to the Doctor, who cannot be possessed; however, shortly afterwards, they find the Doctor's program deactivated. At the suggestion of Tuvok, Janeway decides to divide all command codes between herself and him, believing the entity can only possess one of them at a time.

Torres and Paris discover evidence that points to Tuvok being responsible for several of the events. When these come to light, Tuvok reveals that he is a noncorporeal alien known as the Komar, seeking to bring Voyager's crew to the center of the nebula so that his people can feed on their neural energy; he takes the command crew hostage as he brings the ship about. Suddenly, the warp core is ejected from the ship, halting their progress. Janeway realizes there is a second entity aboard Voyager and Torres confirms that it appears to be Chakotay. The crew overpower the possessed Tuvok and force the Komar entity away from the ship with a magneton pulse.

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Wikipedia