As the U.S.S. Voyager approaches a nebula rich in sirillium, Tuvok is plagued by flashbacks of himself as a boy, trying to rescue a girl who's falling from a cliff. The Doctor suspects Tuvok is experiencing a repressed memory, which can cause permanent brain damage in a Vulcan. Vulcan healing techniques call for the patient to mind-meld with someone he trusts, so that together, the two can bring the memory into the Vulcan's conscious mind. Because of their long-standing friendship, Janeway agrees to meld with Tuvok.
The meld takes Tuvok and Janeway back 80 years to the Vulcan's first assignment on the starship U.S.S. Excelsior, commanded by Hikaru Sulu. Sulu served under the legendary Captain James T. Kirk for many years, so his crew isn't surprised when he disobeys Starfleet orders to try and help Kirk and another old shipmate, Dr. McCoy, who are being held for the murder of the Klingon Chancellor. On the way, the Excelsior passes through a nebula that is similar to the one Voyager spotted prior to Tuvok's first flashback. Once again, Tuvok experiences the memory of the little girl falling, but this time he goes into convulsions.
On Voyager, the Doctor revives Tuvok, and Janeway wonders what the little girl had to do with this period on the Excelsior. All Tuvok can recall is that the Klingons ambushed the starship in the nebula, forcing Sulu to abort the rescue mission.
Reforming the mind-meld, Tuvok relives the Klingon attack. He recalls that during the battle, he aided an injured crewman named Valtane, who died in Tuvok's arms. As he goes through this again, the Doctor notices that the Vulcan is again experiencing erratic brain patterns. Unable to break the mind-meld between Tuvok and Janeway, the Doctor exposes Tuvok's brain to bursts of thoron radiation and in doing so, inadvertently discovers the truth. When Valtane died, a strange alien virus that he'd been carrying in his brain migrated to Tuvok's brain, camouflaging itself as a repressed memory. The Doctor increases the intensity of the radiation and is at last able to destroy the peculiar virus, leaving the mind of Tuvok healthy and Janeway nostalgic for the "old days" when space was still a wild frontier.