Plot summary
Investigating a nebula, Tuvok experiences a flashback of attempting to rescue a young girl off a cliff, then collapses. In Sickbay, he insists the cliff memory never occurred. The Doctor suggests the problem is a repressed memory, which in Vulcans can cause brain damage due to the conflict between the conscious and unconscious minds. He suggests Tuvok initiate a mind meld with a close friend to locate and reintegrate the memory. Tuvok chooses Janeway.
Tuvok initiates multiple mind melds, but instead of the cliff memory they appear in his memories aboard the USS Excelsior serving under Captain Sulu. In several instances the cliff memory resurfaces, rendering Tuvok unconscious. In one instance of the cliff memory, it is not a young Tuvok failing to rescue the girl, but a young Janeway. In Sickbay, the Doctor and Kes deduce the cliff flashback is a false memory created by a virus, and the virus has moved from Tuvok to Janeway. They kill the virus with thoron radiation.
Walking down a corridor, Janeway suggests that Tuvok missed those days serving under Sulu. Tuvok rejects this, but suggests that she could feel nostalgic for the both of them.
Credits
Adapted from Wikipedia contributors, Flashback (Star Trek: Voyager). Paragraphs and formatting adjusted; illustrations, reference markers, block quotations and non-story material omitted.