One was the designation of a Borg drone who was created as a result of a transporter mishap that involved The Doctor's mobile emitter and Seven of Nine's nanoprobes. The mobile emitter, actually a 29th century piece of technology, was heavily damaged during transport and some of the nanoprobes were unknowingly incorporated into its circuitry. The emitter was taken to USS Voyager's science lab where it was subsequently assimilated by Seven's nanoprobes, and in turn began assimilating the lab.
When Mulcahey was sent to check up on the status of the emitter, he was assaulted with extraction tubules from the emitter. However, contrary to standard Borg assimilation procedures, Mulcahey himself was not assimilated or turned into a drone. Instead, the tubules extracted a sample of DNA from the officer. Within the assimilated lab the Borg technology constructed a maturation chamber, where a Borg fetus grew out of the DNA sample. Within hours the fetus had matured into a fully grown Borg drone, a rate of development far out-pacing standard Borg maturation cycles, with the mobile emitter running his neocortex.
As a combined product of 29th century and Borg technology, the new drone possessed several unique features and abilities. These included internal transporter nodes, body armor composed of the same poly-deutonic alloy as used in the mobile emitter, and a multi-spatial personal force field. In essence, it was a 29th century drone, immensely more powerful than 24th century Borg.
Despite the risks, Captain Kathryn Janeway decided to keep this particular drone alive in an attempt to have more help against the Borg, or at least learn more about them. She ordered Seven of Nine to take charge of the drone when it emerged from the maturation chamber and requested its designation and assignment. Unsure what to call the drone, Seven assigned it the designation "One" and set about instructing it on various topics first through a neural link, but as this almost resulted in all of her knowledge taken by the drone, she used Borg data nodes. In short order One was able to assimilate 47 billion teraquads of information. As he learned more, the drone predictably began to inquire about the Borg, and at one point even asked about joining the Collective.
Under Captain Janeway's directions, Seven attempted to dissuade One from seeking further information about the Collective and warned him that joining the Collective would mean losing his individuality. However, One inadvertently alerted a Borg sphere with a second subspace transmitter shortly after his first was deactivated, and it was soon detected approaching Voyager's position. Faced with a confrontation, Janeway ordered Seven to show One the true nature of the Borg. Seven examined the data with One who, despite his desire to experience the hive mind, felt threatened by the Borg's destructive nature and their desire to destroy Voyager and assimilate its crew of individuals.
As the Borg sphere approached and began to tractor in Voyager, One made a decision to help the crew resist the attack. His advanced 29th century technology gave One an advantage and he modified the shields within moments to disengage the tractor. A modification of the phasers was not as successful, and One was forced to board the sphere directly knowing he was superior and they would fail to capture him. Once aboard the sphere One was successful in warding off attacking drones and interfacing himself with the Borg to control the ship's navigation. From there, he propelled the sphere into the same stellar phenomenon that led to his birth, which destroyed the vessel. One was fortunate to escape, creating a shield around himself as he floated in space.
However, his biological components were badly injured. The EMH tried to treat One's injuries, but One refused to allow this, activating his personal force field; he knew the Borg would hunt Voyager to assimilate him for his technology, and One was a grave risk to the ship as long as he was alive. Seven displayed a closeness with One, much like a mother; after One died on the operating table, Seven was greatly saddened by his passing. Afterwards, the Mobile Emitter was retrieved for The Doctor.