When the U.S.S. Voyager encounters a Borg vessel's debris field, sensors also detect a Borg neural interlink frequency. Meanwhile, Seven of Nine hears voices in her head that cause her to change personalities. One minute she is a Klingon hunting for food, and the next she is a little girl wanting to play a game. No one realizes what is happening until Seven, acting as a Klingon, attacks Torres. She is contained in a forcefield but becomes a little girl again. When she suddenly begins speaking Klingon and turns on Tuvok, the Vulcan is forced to stun her with a phaser.
Once in sickbay, Seven's cortical inhibitor is suppressed so she will not react to the voices. The Doctor discovers that Seven's implants are storing new neural patterns of species the Borg assimilated. After arriving at the debris field, the crew finds the source of the interlink frequency — a vinculum. A processing device that interconnects the minds of drones, it is sending a damaged signal to Seven and must be taken off-line. After Janeway agrees to beam it aboard so it can be disabled, Seven finds an alien organism inside that attacks technology by creating a virus.
Seven determines that Species 6339 carried the virus in their bodies and spread it to the Borg cube when assimilated. After her inhibitor begins failing, Seven experiences even more new personalities and must be sedated. Torres and Tuvok target the vinculum's transneural matrix with a dampening field, but it only adapts to the technology and returns to full power. At that point, Seven's own neural pattern is erased.
When Janeway locates a Species 6339 vessel, their leader demands she return the vinculum. They unleashed the virus to spread to all cubes, and they want to return it to the debris field so the Borg will retrieve it. Janeway refuses to do so until Seven has been cured, so Voyager comes under fire from the alien vessel. Meanwhile, Seven's cerebral cortex is under incredible strain, and the Doctor fears that he may never be able to retrieve her neural pattern. Tuvok decides to engage in a Vulcan mind-meld to isolate her true self and guide it to the surface.
As Tuvok enters Seven's chaotic mind, he struggles to find her among the sea of screaming people. He glimpses her being restrained by Klingons and other aliens, but he can't reach her. In Engineering, Torres remodulates the dampening field and finally manages to disable the vinculum. Once it is beamed out to space, the alien vessel ceases its attack. All of the other neural patterns in Seven's mind become dormant, and she and Tuvok return safely from their mind-meld.