A small team from Voyager, including Seven of Nine and the ship's holographic Doctor, are performing a survey of a proto-nebula on a shuttlecraft. When the nebula becomes unstable, the team is evacuated via transport to the ship, but a malfunction briefly merges their patterns together. The malfunction is corrected, and the team is safely transported aboard. The Doctor finds his mobile emitter is failing, and he is safely returned to sickbay while Lt Torres takes the emitter to the science lab to attempt repairs the next day. The crew is unaware that the emitter has gained some of Seven's Borg nanoprobes from the malfunctioning transporter, and it begins assimilating the equipment in the science lab.
Ensign Mulcahy enters the lab the next day, but is knocked unconscious when extraction tubes from the assimilated equipment strike him and take a tissue sample. Seven awakes from her regeneration cycle early and detects that her proximity transceiver has been activated; it happens again while she is in sickbay. The crew is soon aware of a large power draw from the lab, and discover that the nanoprobes have constructed a Borg maturation chamber, rapidly growing a Borg drone from the tissue sample. They determine that the components of the drone are based on the 29th century technology of the emitter, fused in the drone as part of its cerebral cortex; should the Borg of this timeline gain that information, the entire galaxy would be doomed. Captain Janeway orders Seven to disable the Borg transceiver in the drone, but, after struggling with an ethical choice, tells her to allow the drone to mature, hoping to make it understand humanity's individualism once it is out.
The drone (played by J. Paul Boehmer) soon leaves the maturation chamber, and though it immediately attempts to seek out the Borg collective, Seven attempts to convince the drone that he is an individual. She tries to use a direct neural link with the drone to provide it information, but the drone overpowers her control, and attempts to download all the information Seven knows before she manages to put a stop to it. Seven instead enables the drone's linguistic functions, allowing it to learn by language. Several of the crew help to provide the drone with useful information, purposely omitting information about the Borg collective, and soon the drone appears ready to adapt to being an individual, requesting that he be named "One". One recognizes that he is more advanced than any of the technology on Voyager and suspects that he was formed as a result of an accident.
After some time, Seven determines that One is ready to use the Borg regeneration chambers, and they begin the regeneration process together. They are brought out of regeneration early after the crew discovers that One sent a signal to the Borg collective, and a Borg ship is closing in on Voyager; Seven discovers that One's nanoprobes have made a second transceiver they could not detect, and that One himself was not aware of. Seven gives One a "crash course" in the Borg collective, and though One is fascinated by it, Seven asserts that she considers the Voyager crew her collective. One provides the crew with information on how to enhance their shields and weapons based on his advanced design, but this is still not sufficient to prevent the onslaught of a Borg ship. In the middle of battle, One transports over to the Borg ship, interfaces with the collective but resists the attempts to make him part of it, and overrides the controls, sending the ship into the proto-nebula which crushes the ship completely.
The crew are both surprised and saddened by One's sacrifice. They detect a life sign, and find One is still alive, his mechanical parts protected by special force fields but his biological components in danger of dying. He is rushed to sickbay, but he refuses to allow any treatment; One explains that once he connected to the Borg collective, they knew of his presence, and if they knew he were still alive, they would pursue Seven and Voyager endlessly to assimilate him. Instead, his death would protect the crew. Seven attempts to force One to comply to the operation, but he steadfastly refuses, assuring an emotional Seven that she will adapt once he passes away.