During a survey of an expanding proto-nebula, a Class-2 shuttle from Voyager was engulfed by the anomaly's sheerline and lost propulsion. When Voyager attempted to beam the shuttle crew to safety, the crew's transporter patterns briefly merged. Seven of Nine's Borg nanoprobes infected the Doctor's portable holo-emitter during the malfunction, resulting in the EMH being confined to sickbay while the emitter was repaired in Voyager's science lab.
The damaged mobile emitter was left on top of a diagnostic station in the lab, and when the nanoprobes infecting the emitter detected nearby technology, nano-tubules erupted from the emitter and began assimilating and transforming the diagnostic station. Voyager crewman Ensign Mulkahee entered the science lab the next day to discover the presence of a Borg device in the lab, unaware of its nature or how it came to be there. While attempting to scan the device, a tubule erupted from the machine and punctured Mulkahee's skin, extracting a DNA sample.
Both Seven of Nine's Borg tranciever and Voyager's internal sensors detected a Borg presence on the ship and so Seven and a security team went to the science lab to investigate. Seven soon deduced that the diagnostic console had been turned into a maturation chamber, a device that accelerates the growth of Borg drones that are assimilated during childhood. The chamber was protected by a forcefield and possessed components that were non-standard to the Borg. Seven was able to breach the chamber's forcefield since the machine recognised her as Borg and upon closer inspection she found that a human foetus was developing inside it. The foetus had been cloned from Ensign Mulkahee's DNA and the maturation chamber and the foetus's cybernetics were composed of materials similar to those found in the doctor's mobile emitter.
The drone developed at an accelerated rate, growing from a foetus to the size of a six-year old boy in just over an hour. As the drone grew, the maturation chamber reconfigured itself to accomodate the size of its occupant. The drone's implants developed just as quickly as its organic systems. By the time its maturation cycle had finished, it was covered in polynutronic body armour from the neck down. It possessed superior technological systems far beyond those of normal Borg drones, including multi-spatial shielding that could be activated without recieving prior attack from a weapon, and internal transporter nodes that would allow it to teleport under its own power.
After completing its maturation cycle, the drone emerged from its maturation chamber and requested instructions and a designation. Verbal communication with it seemed impossible as its responses were programmed, and so Seven of Nine attempted a direct neural interface, injecting the drone with her assimilation tubule. This activated its cognitive functions and allowed it to assimilate data on its own. The drone became self-aware and began asking Seven and other crew members many questions as to how and why it was here as well as questions about Voyager and its occupants. It also requested a designation and began referring to itself as "One" following a conversation with Neelix, who suggested that he choose a name to define himself since there was "only one of him".
As One interacted with the Voyager crew, it learned about the ship's systems and quickly became a master in several scientific and social subjects, and even assisted the engineering staff in studying the nearby nebula that was continuing to expand. However, there was also cause for concern: One expressed much curiosity regarding the Borg, which Seven actively avoided discussing with him in order to keep him from being drawn to the collective. Unfortunately, One's neural implants adapted a secondary tranciever device while he was regenerating in his alcove and a proximity signal was broadcast that a Borg ship detected, alerting the collective to the super-drone's existence.
When Captain Kathryn Janeway and the Voyager crew became aware of One's communication with the Borg, they revealed to him the answers to all his questions regarding the collective, explaining their forced assimilation of countless sentient beings and the unfathomable destruction they left in their wake. Captain Janeway warned One that the Borg would only become even deadlier were they to assimilate him and requested that he use his advanced abilities to help enhance Voyager's tactical systems.
A Borg sphere later arrived and a battle ensued. One was able to remodulate Voyager's shields to stop the Borg's tractor beams from locking on and enhanced the ship's phasers to penetrate the sphere's armour. The sphere's defences were too great and it was able to feedback Voyager's phaser strikes. One was unable to enhance Voyager's systems any further and decided that he could defeat the Borg by boarding the sphere and interfacing with it. Using his personal transporter nodes, One beamed on to the Borg sphere, repelling the attacking drones with his body shields and forcefully interfacing with an alcove, hacking into the ship's propulsion systems and flying them into the nearby nebula. The extreme gravimetric forces of the nebula crushed the sphere's hull, causing it to implode, but One narrowly survived by erecting a multi-spatial forcefield around himself.
One was beamed directly into Voyager's medical bay, but had suffered tremendous damage to his organic systems. One refused to undergo surgery, claiming he should not be allowed to exist. Since the Borg were now aware of him, One was making Voyager a prime target and chose to die rather than endanger the ship. He used the last of his energy to shield his body, preventing the Doctor from treating him. Seven of Nine tried ordering One to comply, saying that he was hurting her. One merely replied "You will adapt..." His systems then shut down and he died.