Continuing on course for home, Voyager's long-range sensors detect a debris field from a Borg cube. The crew decides to alter course to avoid the debris, since the Borg tend to salvage materials and parts from their vessels, and may return to the area.
As their general course takes them closer to the Borg vessel's debris field, Seven begins experiencing a form of multiple personality disorder and alternate personalities begin to manifest themselves. After several incidents witnessed by the crew, she is taken to Sickbay to try to find the source of the neurological problem. Seven speculates that it could be due to an active and undamaged vinculum (transponder device) still in the debris field from the Borg vessel. Voyager alters course to retrieve the vinculum and investigate further. Once at the debris field, they find the vinculum, still active, and beam it aboard for inspection.
Further scans reveal a synthetic pathogenic virus inhabiting it. After reviewing a number of data cubes, they deduce that the virus must have been introduced by the Borg cube's last assimilation, a small vessel carrying aliens referred to by the Borg as Species 6339. Voyager searches out these alien travelers, who reveal their attempt to infect the Borg Collective with a virus designed to shut it down. Thirteen of their people sacrificed themselves to be assimilated so they could spread the virus. Part of their plan depends on more Borg to find the vinculum and use it like a "Typhoid Mary" to spread the virus further into the Borg network.
However, Captain Janeway wants to keep the vinculum for a short period of time while the crew attempts to separate Seven from it. Seven struggles to maintain control of the personalities splitting her consciousness - including some with memories of the defeat of the Starfleet battle group at Wolf 359 - while Chief Engineer B'Elanna Torres and her team begin the difficult task of shutting down the vinculum. Tuvok suggests a Vulcan mind meld to help calm Seven. The aliens protest the destruction of the vinculum and initiate a battle with Voyager, but B'Elanna is successful and Seven is freed from the link.