As Voyager monitors a nebula with large quantities of the power source sirillium, Tuvok finds his hands shaking, and requests permission to go to sickbay. En route, he experiences a flashback that involves him trying to pull a young girl off a cliff to safety. The girl ends up falling, horrifying young Tuvok. Eventually, he arrives in sickbay, and collapses, convulsing on the floor.
In sickbay, Tuvok describes the memory, but insists the events he 'remembered' never occurred. With no clear cause of his symptoms, he is dismissed with a device to monitor his readings in case such an event occurs again. That night, he attempts to build a 'structure of harmony' in an effort to aid his meditation, but cannot stop the structure from collapsing. After a visit from Kes, he attempts again.
The next morning, he notes he had spent fourteen hours meditating, and still had no idea what the cause of the problem was. Ensign Kim could not find anything unusual with the nebula they were near, but Tuvok suggests they scan it for Klingon activity. B'Elanna Torres reminds him that the Klingon Empire is on the other side of the galaxy. Moments later, Tuvok sees the mental image again, and collapses once more.
In sickbay, 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. The only possible course of action is to initiate a mind meld â normally undertaken with a family member. Tuvok explains he would go to one of the Vulcans on board but he feels Janeway is the best choice to assist him in locating and reintegrating the memory. During the meld she would be an outside observer of the memories, unnoticed by anyone but Tuvok.
When Tuvok initiates the mind meld, he attempts to take them to the cliff in his memory, but instead they appear on the USS Excelsior, under attack by the Klingons. Explaining the attack, the memory moves once more, to three days before, when Praxis exploded. After learning about how Captain Kirk and Dr McCoy were placed on trial for the attempted assassination of the Klingon Chancellor, Captain Sulu attempted a rescue mission, which Tuvok was the only Excelsior crew member to protest.
In an effort to avoid confrontation with the Klingons, Sulu ordered an approach path through a nebula which bore a remarkable physical similarity to the one Voyager was observing in the Delta Quadrant â the appearance causing Tuvok to recall the repressed memory once more, breaking the mind meld.
After a period of recovery for Tuvok, during which Janeway and Kim comment on the differences between Starfleet of the 23rd century and the 24th, they try the mind meld again â appearing once more on the Excelsior. After a discussion about Tuvok's motivations for joining Starfleet, a small Klingon attack began. After Sulu claimed their navigational equipment had malfunctioned, Kang insisted upon escorting the Excelsior back to Federation space, to help them from getting lost again. Sulu agreed, but on the way out of the nebula they were in, came up with a plan to disable the Klingon ship by igniting the sirillium that was also present in that nebula. After this succeeded, they set course once more for Qo'noS, before being attacked again by three Klingon battlecruisers â an attack that killed Lieutenant Dimitri Valtane. As Tuvok watched Valtane die, the memory appeared once more, and in sickbay, the neural engrams destabilize, preventing the meld from being broken. On the Excelsior, Sulu could suddenly see Janeway, who was supposed to merely be an observer. In an effort to blend into the memories, Tuvok takes Janeway to a time where she can steal Commander Janice Rand's uniform. In sickbay, the Doctor and Kes notice an irregularity in the memories, and deduced they were not in fact memories, but instead a virus. Using thoron radiation, they begin to kill the virus.
On the Excelsior, the attack occurs once more, and Valtane dies when a plasma conduit behind his console explodes. The image of the girl on the cliff appears, but this time, it is Janeway who is letting the girl fall. As the Doctor continues the efforts to kill the virus, it tracks back, changing to Valtane, and then an endless stream of other children. Eventually, it dies, and Tuvok breaks the meld.
In sickbay, the Doctor and Kes explain what must have happened: the virus thrived on neural peptides, and hid itself by creating the false memory that the person bearing it would repress, so the virus could live in secret, and migrate from person to person as its hosts died.
Walking down a corridor, Janeway suggests that Tuvok missed those days, a suggestion Tuvok rejects. However, he admits that he is pleased to have been a part of them, and having experienced the memories, Janeway says she feels she was a part of them as well. As a result, Tuvok suggests that she could feel nostalgic for the both of them.