As his away team is beamed back to Voyager, some parts of the Doctor's mobile emitter fuse together, so Torres takes it to the science lab. Unseen, the emitter sprouts Borg implants. When Ensign Mulcahey enters the lab the next morning, he is attacked by extraction tubules. Meanwhile, Seven of Nine's proximity transceiver is activated, alerting her to a Borg presence on the ship. She finds Mulcahey in the lab, along with a Borg maturation chamber containing a fetal drone.

Seven determines that when she and the Doctor were transported, some of her nanoprobes infected his emitter. It was assimilated, and the Borg used Mulcahey's genetic code to create a lifeform. As it continues to mature at a rapid rate, the drone is observed to be mostly human, with many implants. The emitter is part of his central nervous system and gives him 29th-century technology. His connection with the Collective is dampened, and Seven attempts a neural interface to give him instructions. Instead, he tries to assimilate her knowledge.

Because a neural link is too dangerous, the drone, who chooses "One" as his designation, uses Borg data nodes to assimilate information. He quickly absorbs knowledge of the ship's systems and begins expressing curiosity about the Borg. Seven fears he will be tempted to seek out the Collective and refuses to answer his questions. As she and One regenerate in their alcoves, the Borg pick up One's signal and set a course to intercept Voyager.

When Voyager detects the approaching Borg, Seven discovers that One's cranial implants created a secondary transceiver to signal them. Janeway and Seven have no choice but to describe to One the Collective's destructive mentality and explain that with his technology, the Borg would be even more dangerous. The Captain asks him to help strengthen Voyager's defenses, but One is confused. He is intrigued by the Collective, yet he senses its evil. There is no time left to discuss the matter, however, as a Borg ship approaches.

One hears the voice of the Collective, but he chooses to save Voyager from destruction. He beams to the sphere to interface directly and disrupt it from within. When his superior technology accesses the navigation system, he steers the sphere into a nebula, destroying it. One survives and is beamed to sickbay, where he refuses to let the Doctor operate to save him because he knows the Borg will keep pursuing him. He doesn't want to put the crew in danger, so he allows himself to die.