Stardate 50126.4: Voyager is about to enter a nebula when Tuvok suddenly experiences dizziness and then a flashback of a girl falling from a precipice. The Doctor does not know how to treat the apparent repressed memory in a Vulcan brain. So Tuvok performs a mind-meld with Janeway, with the goal that she, as an observer of his memories, can help him repair the damage. After establishing the link, Tuvok and Janeway are surprised that they don't find themselves on the precipice but on the bridge of the USS Excelsior during the Praxis Crisis on Stardate 9521, 80 years ago. In the course of Captain Sulu's attempt to save Captain Kirk and Doctor McCoy, the Excelsior traverses a nebula that looks much like the one in the Delta Quadrant, so the latter may have been the trigger for the memory to resurface. Still, the origin of the memory remains unknown. In a second mind-meld, Tuvok relives the death of his bunkmate Dmitri Valtane, who is killed in a Klingon attack after leaving that nebula. The Doctor and Kes are alarmed as brain damage to Tuvok and Janeway is imminent and they find no way to terminate the mind-meld. In the mind-meld, Tuvok's memory of the past gets distorted, and Janeway becomes an active participant. The two go back to a time before Valtane's death to get another chance to investigate what happened at that moment. It turns out that a viral parasite that inhabited Valtane transferred itself to Tuvok when Valtane died. It camouflaged itself as a traumatic memory in order not to be attacked by the body's immune system. The Doctor too notices what is going on and kills the virus with thoron radiation.

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